diff --git a/Backend/src/services/ChaptersLesson.service.ts b/Backend/src/services/ChaptersLesson.service.ts
index 41b401c6..685642be 100644
--- a/Backend/src/services/ChaptersLesson.service.ts
+++ b/Backend/src/services/ChaptersLesson.service.ts
@@ -483,10 +483,22 @@ export class ChaptersLessonService {
             if (currentLesson.chapter_id !== chapter_id) {
                 throw new NotFoundError('Lesson not found in this chapter');
             }
-            const oldSortOrder = currentLesson.sort_order;
+
+            // First, normalize sort_order to fix any gaps or duplicates
+            await this.normalizeLessonSortOrder(chapter_id);
+
+            // Re-fetch the lesson to get updated sort_order after normalization
+            const normalizedLesson = await prisma.lesson.findUnique({ where: { id: lesson_id } });
+            if (!normalizedLesson) throw new NotFoundError('Lesson not found');
+
+            const oldSortOrder = normalizedLesson.sort_order;
+
+            // Get total lesson count to validate sort_order (1-based)
+            const lessonCount = await prisma.lesson.count({ where: { chapter_id } });
+            const validNewSortOrder = Math.max(1, Math.min(newSortOrder, lessonCount));
 
             // If same position, no need to reorder
-            if (oldSortOrder === newSortOrder) {
+            if (oldSortOrder === validNewSortOrder) {
                 const lessons = await prisma.lesson.findMany({
                     where: { chapter_id },
                     orderBy: { sort_order: 'asc' }
@@ -495,12 +507,12 @@ export class ChaptersLessonService {
             }
 
             // Shift other lessons to make room for the insert
-            if (oldSortOrder > newSortOrder) {
+            if (oldSortOrder > validNewSortOrder) {
                 // Moving up: shift lessons between newSortOrder and oldSortOrder-1 down by 1
                 await prisma.lesson.updateMany({
                     where: {
                         chapter_id,
-                        sort_order: { gte: newSortOrder, lt: oldSortOrder }
+                        sort_order: { gte: validNewSortOrder, lt: oldSortOrder }
                     },
                     data: { sort_order: { increment: 1 } }
                 });
@@ -509,14 +521,14 @@ export class ChaptersLessonService {
                 await prisma.lesson.updateMany({
                     where: {
                         chapter_id,
-                        sort_order: { gt: oldSortOrder, lte: newSortOrder }
+                        sort_order: { gt: oldSortOrder, lte: validNewSortOrder }
                     },
                     data: { sort_order: { decrement: 1 } }
                 });
             }
 
             // Update the target lesson to the new position
-            await prisma.lesson.update({ where: { id: lesson_id }, data: { sort_order: newSortOrder } });
+            await prisma.lesson.update({ where: { id: lesson_id }, data: { sort_order: validNewSortOrder } })
 
             // Fetch all lessons with updated order
             const lessons = await prisma.lesson.findMany({
@@ -576,10 +588,16 @@ export class ChaptersLessonService {
                 }
             }
 
+            // Get chapter_id before deletion for normalization
+            const chapterId = lesson.chapter_id;
+
             // Delete lesson (CASCADE will delete: attachments, quiz, questions, choices)
             // Based on Prisma schema: onDelete: Cascade
             await prisma.lesson.delete({ where: { id: lesson_id } });
 
+            // Normalize sort_order for remaining lessons (fill gaps)
+            await this.normalizeLessonSortOrder(chapterId);
+
             return { code: 200, message: 'Lesson deleted successfully' };
         } catch (error) {
             logger.error(`Error deleting lesson: ${error}`);
@@ -1219,6 +1237,34 @@ export class ChaptersLessonService {
         logger.info(`Normalized sort_order for quiz ${quizId}: ${questions.length} questions`);
     }
 
+    /**
+     * Normalize sort_order for all lessons in a chapter
+     * Ensures sort_order is sequential starting from 1 with no gaps or duplicates
+     * 
+     * @param chapterId Chapter ID to normalize
+     */
+    private async normalizeLessonSortOrder(chapterId: number): Promise<void> {
+        // Get all lessons ordered by current sort_order
+        const lessons = await prisma.lesson.findMany({
+            where: { chapter_id: chapterId },
+            orderBy: { sort_order: 'asc' },
+            select: { id: true, sort_order: true }
+        });
+
+        if (lessons.length === 0) return;
+
+        // Update each lesson with sequential sort_order starting from 1
+        const updates = lessons.map((lesson, index) =>
+            prisma.lesson.update({
+                where: { id: lesson.id },
+                data: { sort_order: index + 1 }
+            })
+        );
+
+        await prisma.$transaction(updates);
+        logger.info(`Normalized sort_order for chapter ${chapterId}: ${lessons.length} lessons`);
+    }
+
     /**
      * อัพเดตการตั้งค่า Quiz
      * Update quiz settings (title, passing_score, time_limit, etc.)