diff --git a/Backend/src/services/ChaptersLesson.service.ts b/Backend/src/services/ChaptersLesson.service.ts
index ae75acf7..41b401c6 100644
--- a/Backend/src/services/ChaptersLesson.service.ts
+++ b/Backend/src/services/ChaptersLesson.service.ts
@@ -1032,11 +1032,23 @@ export class ChaptersLessonService {
                 throw new NotFoundError('Question not found in this quiz');
             }
 
-            const oldSortOrder = existingQuestion.sort_order;
             const quizId = lesson.quiz.id;
 
+            // First, normalize sort_order to fix any gaps or duplicates
+            await this.normalizeQuestionSortOrder(quizId);
+
+            // Re-fetch the question to get updated sort_order after normalization
+            const normalizedQuestion = await prisma.question.findUnique({ where: { id: question_id } });
+            if (!normalizedQuestion) throw new NotFoundError('Question not found');
+
+            const oldSortOrder = normalizedQuestion.sort_order;
+
+            // Get total question count to validate sort_order (1-based)
+            const questionCount = await prisma.question.count({ where: { quiz_id: quizId } });
+            const newSortOrder = Math.max(1, Math.min(sort_order, questionCount));
+
             // If same position, no need to reorder
-            if (oldSortOrder === sort_order) {
+            if (oldSortOrder === newSortOrder) {
                 const questions = await prisma.question.findMany({
                     where: { quiz_id: quizId },
                     orderBy: { sort_order: 'asc' },
@@ -1046,12 +1058,12 @@ export class ChaptersLessonService {
             }
 
             // Shift other questions to make room for the insert
-            if (oldSortOrder > sort_order) {
+            if (oldSortOrder > newSortOrder) {
                 // Moving up: shift questions between newSortOrder and oldSortOrder-1 down by 1
                 await prisma.question.updateMany({
                     where: {
                         quiz_id: quizId,
-                        sort_order: { gte: sort_order, lt: oldSortOrder }
+                        sort_order: { gte: newSortOrder, lt: oldSortOrder }
                     },
                     data: { sort_order: { increment: 1 } }
                 });
@@ -1060,7 +1072,7 @@ export class ChaptersLessonService {
                 await prisma.question.updateMany({
                     where: {
                         quiz_id: quizId,
-                        sort_order: { gt: oldSortOrder, lte: sort_order }
+                        sort_order: { gt: oldSortOrder, lte: newSortOrder }
                     },
                     data: { sort_order: { decrement: 1 } }
                 });
@@ -1069,7 +1081,7 @@ export class ChaptersLessonService {
             // Update the question's sort order
             await prisma.question.update({
                 where: { id: question_id },
-                data: { sort_order }
+                data: { sort_order: newSortOrder }
             });
 
             // Fetch all questions with updated order
@@ -1121,6 +1133,9 @@ export class ChaptersLessonService {
             // Delete the question (CASCADE will delete choices)
             await prisma.question.delete({ where: { id: question_id } });
 
+            // Normalize sort_order for remaining questions (fill gaps)
+            await this.normalizeQuestionSortOrder(lesson.quiz.id);
+
             // Recalculate scores for remaining questions
             await this.recalculateQuestionScores(lesson.quiz.id);
 
@@ -1176,6 +1191,34 @@ export class ChaptersLessonService {
         logger.info(`Recalculated quiz ${quizId}: ${questionCount} questions. Base: ${baseScore}, Remainder: ${remainder}`);
     }
 
+    /**
+     * Normalize sort_order for all questions in a quiz
+     * Ensures sort_order is sequential starting from 0 with no gaps or duplicates
+     * 
+     * @param quizId Quiz ID to normalize
+     */
+    private async normalizeQuestionSortOrder(quizId: number): Promise<void> {
+        // Get all questions ordered by current sort_order
+        const questions = await prisma.question.findMany({
+            where: { quiz_id: quizId },
+            orderBy: { sort_order: 'asc' },
+            select: { id: true, sort_order: true }
+        });
+
+        if (questions.length === 0) return;
+
+        // Update each question with sequential sort_order starting from 1
+        const updates = questions.map((question, index) =>
+            prisma.question.update({
+                where: { id: question.id },
+                data: { sort_order: index + 1 }
+            })
+        );
+
+        await prisma.$transaction(updates);
+        logger.info(`Normalized sort_order for quiz ${quizId}: ${questions.length} questions`);
+    }
+
     /**
      * อัพเดตการตั้งค่า Quiz
      * Update quiz settings (title, passing_score, time_limit, etc.)