diff --git a/app/Http/Controllers/PurchaseOrderController.php b/app/Http/Controllers/PurchaseOrderController.php
index 9f40598..26ea08a 100644
--- a/app/Http/Controllers/PurchaseOrderController.php
+++ b/app/Http/Controllers/PurchaseOrderController.php
@@ -97,8 +97,8 @@ class PurchaseOrderController extends Controller
             'items' => 'required|array|min:1',
             'items.*.productId' => 'required|exists:products,id',
             'items.*.quantity' => 'required|numeric|min:0.01',
-            'items.*.unitPrice' => 'required|numeric|min:0',
-            'items.*.unitId' => 'nullable|exists:units,id', // 驗證單位ID
+            'items.*.subtotal' => 'required|numeric|min:0', // 總金額
+            'items.*.unitId' => 'nullable|exists:units,id',
         ]);
 
         try {
@@ -122,7 +122,7 @@ class PurchaseOrderController extends Controller
 
             $totalAmount = 0;
             foreach ($validated['items'] as $item) {
-                $totalAmount += $item['quantity'] * $item['unitPrice'];
+                $totalAmount += $item['subtotal'];
             }
 
             // Simple tax calculation (e.g., 5%)
@@ -157,12 +157,15 @@ class PurchaseOrderController extends Controller
             ]);
 
             foreach ($validated['items'] as $item) {
+                // 反算單價
+                $unitPrice = $item['quantity'] > 0 ? $item['subtotal'] / $item['quantity'] : 0;
+
                 $order->items()->create([
                     'product_id' => $item['productId'],
                     'quantity' => $item['quantity'],
-                    'unit_id' => $item['unitId'] ?? null, // 儲存單位ID
-                    'unit_price' => $item['unitPrice'],
-                    'subtotal' => $item['quantity'] * $item['unitPrice'],
+                    'unit_id' => $item['unitId'] ?? null, 
+                    'unit_price' => $unitPrice,
+                    'subtotal' => $item['subtotal'],
                 ]);
             }
 
@@ -310,8 +313,8 @@ class PurchaseOrderController extends Controller
             'items' => 'required|array|min:1',
             'items.*.productId' => 'required|exists:products,id',
             'items.*.quantity' => 'required|numeric|min:0.01',
-            'items.*.unitPrice' => 'required|numeric|min:0',
-            'items.*.unitId' => 'nullable|exists:units,id', // 驗證單位ID
+            'items.*.subtotal' => 'required|numeric|min:0', // 總金額
+            'items.*.unitId' => 'nullable|exists:units,id',
         ]);
 
         try {
@@ -319,7 +322,7 @@ class PurchaseOrderController extends Controller
 
             $totalAmount = 0;
             foreach ($validated['items'] as $item) {
-                $totalAmount += $item['quantity'] * $item['unitPrice'];
+                $totalAmount += $item['subtotal'];
             }
 
             // Simple tax calculation (e.g., 5%)
@@ -340,12 +343,15 @@ class PurchaseOrderController extends Controller
             // Sync items
             $order->items()->delete();
             foreach ($validated['items'] as $item) {
+                // 反算單價
+                $unitPrice = $item['quantity'] > 0 ? $item['subtotal'] / $item['quantity'] : 0;
+
                 $order->items()->create([
                     'product_id' => $item['productId'],
                     'quantity' => $item['quantity'],
-                    'unit_id' => $item['unitId'] ?? null, // 儲存單位ID
-                    'unit_price' => $item['unitPrice'],
-                    'subtotal' => $item['quantity'] * $item['unitPrice'],
+                    'unit_id' => $item['unitId'] ?? null, 
+                    'unit_price' => $unitPrice,
+                    'subtotal' => $item['subtotal'],
                 ]);
             }
 
diff --git a/resources/js/Components/PurchaseOrder/PurchaseOrderItemsTable.tsx b/resources/js/Components/PurchaseOrder/PurchaseOrderItemsTable.tsx
index e1d7912..8da4164 100644
--- a/resources/js/Components/PurchaseOrder/PurchaseOrderItemsTable.tsx
+++ b/resources/js/Components/PurchaseOrder/PurchaseOrderItemsTable.tsx
@@ -50,8 +50,7 @@ export function PurchaseOrderItemsTable({
                         <TableHead className="w-[10%] text-left">數量</TableHead>
                         <TableHead className="w-[12%] text-left">單位</TableHead>
                         <TableHead className="w-[12%] text-left">換算基本單位</TableHead>
-                        <TableHead className="w-[12%] text-left">金額</TableHead>
-                        <TableHead className="w-[12%] text-left">小計</TableHead>
+                        <TableHead className="w-[15%] text-left">總金額</TableHead>
                         <TableHead className="w-[15%] text-left">單價 / 基本單位</TableHead>
                         {!isReadOnly && <TableHead className="w-[5%]"></TableHead>}
                     </TableRow>
@@ -60,7 +59,7 @@ export function PurchaseOrderItemsTable({
                     {items.length === 0 ? (
                         <TableRow>
                             <TableCell
-                                colSpan={isReadOnly ? 7 : 8}
+                                colSpan={isReadOnly ? 6 : 7}
                                 className="text-center text-gray-400 py-12 italic"
                             >
                                 {isDisabled ? "請先選擇供應商後才能新增商品" : "尚未新增任何商品項"}
@@ -69,9 +68,12 @@ export function PurchaseOrderItemsTable({
                     ) : (
                         items.map((item, index) => {
                             // 計算換算後的單價 (基本單位單價)
+                            // unitPrice is derived from subtotal / quantity
+                            const currentUnitPrice = item.unitPrice;
+
                             const convertedUnitPrice = item.selectedUnit === 'large' && item.conversion_rate
-                                ? item.unitPrice / item.conversion_rate
-                                : item.unitPrice;
+                                ? currentUnitPrice / item.conversion_rate
+                                : currentUnitPrice;
 
                             return (
                                 <TableRow key={index}>
@@ -162,44 +164,39 @@ export function PurchaseOrderItemsTable({
                                         </div>
                                     </TableCell>
 
-                                    {/* 單價 */}
+                                    {/* 總金額 (主要輸入欄位) */}
                                     <TableCell className="text-left">
                                         {isReadOnly ? (
-                                            <span className="font-medium text-gray-900">{formatCurrency(item.unitPrice)}</span>
+                                            <span className="font-bold text-primary">{formatCurrency(item.subtotal)}</span>
                                         ) : (
                                             <div className="space-y-1">
                                                 <Input
                                                     type="number"
                                                     min="0"
-                                                    step="0.1"
-                                                    value={item.unitPrice || ""}
+                                                    step="1"
+                                                    value={item.subtotal || ""}
                                                     onChange={(e) =>
-                                                        onItemChange?.(index, "unitPrice", Number(e.target.value))
+                                                        onItemChange?.(index, "subtotal", Number(e.target.value))
                                                     }
                                                     disabled={isDisabled}
                                                     className={`h-10 text-left w-32 ${
-                                                        // 如果有數量但沒有單價,顯示錯誤樣式
-                                                        item.quantity > 0 && (!item.unitPrice || item.unitPrice <= 0)
+                                                        // 如果有數量但沒有金額,顯示錯誤樣式
+                                                        item.quantity > 0 && (!item.subtotal || item.subtotal <= 0)
                                                             ? "border-red-400 bg-red-50 focus-visible:ring-red-500"
                                                             : "border-gray-200"
                                                         }`}
                                                 />
-                                                {/* 錯誤提示 (保留必填提示) */}
-                                                {item.quantity > 0 && (!item.unitPrice || item.unitPrice <= 0) && (
+                                                {/* 錯誤提示 */}
+                                                {item.quantity > 0 && (!item.subtotal || item.subtotal <= 0) && (
                                                     <p className="text-[10px] text-red-600 font-medium">
-                                                        ❌ 請填寫金額
+                                                        ❌ 請填寫總金額
                                                     </p>
                                                 )}
                                             </div>
                                         )}
                                     </TableCell>
 
-                                    {/* 小計 */}
-                                    <TableCell className="text-left">
-                                        <span className="font-bold text-primary">{formatCurrency(item.subtotal)}</span>
-                                    </TableCell>
-
-                                    {/* 換算採購單價 / 基本單位 */}
+                                    {/* 換算採購單價 / 基本單位 (顯示換算結果) */}
                                     <TableCell className="text-left">
                                         <div className="flex flex-col">
                                             <div className="text-gray-500 font-medium text-sm">
diff --git a/resources/js/Pages/PurchaseOrder/Create.tsx b/resources/js/Pages/PurchaseOrder/Create.tsx
index 525e927..99c37ba 100644
--- a/resources/js/Pages/PurchaseOrder/Create.tsx
+++ b/resources/js/Pages/PurchaseOrder/Create.tsx
@@ -115,6 +115,7 @@ export default function CreatePurchaseOrder({
                 quantity: item.quantity,
                 unitPrice: item.unitPrice,
                 unitId: item.unitId,
+                subtotal: item.subtotal,
             })),
         };
 
diff --git a/resources/js/hooks/usePurchaseOrderForm.ts b/resources/js/hooks/usePurchaseOrderForm.ts
index 516dcb9..47e139a 100644
--- a/resources/js/hooks/usePurchaseOrderForm.ts
+++ b/resources/js/hooks/usePurchaseOrderForm.ts
@@ -87,7 +87,6 @@ export function usePurchaseOrderForm({ order, suppliers }: UsePurchaseOrderFormP
                 item.previousPrice = product.lastPrice;
 
                 // 決定預設單位
-                // 若有採購單位且等於大單位，預設為大單位
                 const isPurchaseUnitLarge = product.purchase_unit_id && product.large_unit_id && product.purchase_unit_id === product.large_unit_id;
 
                 if (isPurchaseUnitLarge) {
@@ -97,6 +96,9 @@ export function usePurchaseOrderForm({ order, suppliers }: UsePurchaseOrderFormP
                     item.selectedUnit = 'base';
                     item.unitId = product.base_unit_id;
                 }
+
+                // 初始小計 = 數量 * 單價
+                item.subtotal = calculateSubtotal(Number(item.quantity), Number(item.unitPrice));
             }
         } else if (field === "selectedUnit") {
             // @ts-ignore
@@ -106,17 +108,24 @@ export function usePurchaseOrderForm({ order, suppliers }: UsePurchaseOrderFormP
             } else {
                 item.unitId = item.base_unit_id;
             }
+            // Switch unit doesn't change Total Amount (Subtotal), but implies Unit Price changes?
+            // Actually if I switch unit, the Quantity is usually for that unit.
+            // If I have 1 Box ($100), and switch to Pc. Quantity is still 1. 
+            // Total is $100. So Unit Price (per Pc) becomes $100.
+            // This seems safely consistent with "Total Amount" anchor.
         } else {
             // @ts-ignore
             item[field] = value;
         }
 
-        // 計算小計
-        if (field === "quantity" || field === "unitPrice" || field === "productId") {
-            item.subtotal = calculateSubtotal(
-                Number(item.quantity),
-                Number(item.unitPrice)
-            );
+        // 重新計算 (Always derive UnitPrice from Subtotal and Quantity)
+        // 除了剛剛已經算過 subtotal 的 productId case
+        if (field !== "productId") {
+            if (item.quantity > 0) {
+                item.unitPrice = Number(item.subtotal) / Number(item.quantity);
+            } else {
+                item.unitPrice = 0;
+            }
         }
 
         newItems[index] = item;