diff --git a/src/main/java/com/kamco/cd/kamcoback/upload/service/UploadService.java b/src/main/java/com/kamco/cd/kamcoback/upload/service/UploadService.java
index 0c018467..7372b7e2 100644
--- a/src/main/java/com/kamco/cd/kamcoback/upload/service/UploadService.java
+++ b/src/main/java/com/kamco/cd/kamcoback/upload/service/UploadService.java
@@ -108,20 +108,48 @@ public class UploadService {
     if (Objects.equals(chunkIndex, chunkTotalIndex)) {
 
       upAddReqDto.setUploadId(dto.getUploadId());
-      upAddReqDto.setStatus(FileUploadStatus.DONE.name());
+      upAddReqDto.setStatus(FileUploadStatus.DONE.name()); // DONE 말고 MERGING 추천
       uploadSessionCoreService.updateUploadSessionStatus(upAddReqDto);
 
+      log.info(
+          "merge start: uploadId={}, fileName={}, chunkIndex={}, chunkTotalIndex={}, tmpDir={}, finalDir={}",
+          dto.getUploadId(),
+          fileName,
+          chunkIndex,
+          chunkTotalIndex,
+          tmpDataSetDir,
+          fianlDir);
+
       try {
         this.mergeChunks(tmpDataSetDir, fianlDir, fileName, chunkTotalIndex);
 
-        upAddReqDto.setUploadId(dto.getUploadId());
         upAddReqDto.setStatus("MERGED");
         uploadSessionCoreService.updateUploadSessionStatus(upAddReqDto);
 
+        log.info(
+            "merge success: uploadId={}, fileName={}, outputDir={}",
+            dto.getUploadId(),
+            fileName,
+            fianlDir);
+
       } catch (IOException e) {
-        // throw new RuntimeException(e);
+
+        // 실패 상태도 남기는 걸 강추
+        upAddReqDto.setStatus("MERGE_FAIL");
+        uploadSessionCoreService.updateUploadSessionStatus(upAddReqDto);
+
+        log.error(
+            "merge failed: uploadId={}, fileName={}, chunkIndex={}, chunkTotalIndex={}, tmpDir={}, finalDir={}",
+            dto.getUploadId(),
+            fileName,
+            chunkIndex,
+            chunkTotalIndex,
+            tmpDataSetDir,
+            fianlDir,
+            e);
+
         upRes.setRes("fail");
-        upRes.setResMsg("파일Chunk 병합(merge) 애러");
+        upRes.setResMsg("파일Chunk 병합(merge) 오류");
         return upRes;
       }
     }
@@ -212,28 +240,64 @@ public class UploadService {
     return dto;
   }
 
-  public void mergeChunks(String tmpDir, String fianlDir, String fileName, int chunkTotalIndex)
+  public void mergeChunks(String tmpDir, String finalDir, String fileName, int chunkTotalIndex)
       throws IOException {
+    long start = System.currentTimeMillis();
+    Path outputPath = Paths.get(finalDir, fileName);
+
+    log.info(
+        "mergeChunks start: fileName={}, tmpDir={}, outputPath={}, lastChunkIndex={}",
+        fileName,
+        tmpDir,
+        outputPath,
+        chunkTotalIndex);
+
+    long totalBytes = 0;
 
-    Path outputPath = Paths.get(fianlDir, fileName);
     try (FileChannel outChannel =
-        FileChannel.open(outputPath, StandardOpenOption.CREATE, StandardOpenOption.WRITE)) {
+        FileChannel.open(
+            outputPath,
+            StandardOpenOption.CREATE,
+            StandardOpenOption.WRITE,
+            StandardOpenOption.TRUNCATE_EXISTING)) {
+
       for (int i = 0; i <= chunkTotalIndex; i++) {
-        Path chunkPath = Paths.get(tmpDir, i + "");
+        Path chunkPath = Paths.get(tmpDir, String.valueOf(i));
 
         try (FileChannel inChannel = FileChannel.open(chunkPath, StandardOpenOption.READ)) {
-          long transferred = 0;
           long size = inChannel.size();
+          long transferred = 0;
           while (transferred < size) {
             transferred += inChannel.transferTo(transferred, size - transferred, outChannel);
           }
+          totalBytes += size;
         }
-        // 병합 후 즉시 삭제하여 디스크 공간 확보
+
         Files.delete(chunkPath);
       }
+
+      try {
+        FIleChecker.deleteFolder(tmpDir);
+      } catch (Exception e) {
+        log.warn("tmpDir delete failed (merge already succeeded): tmpDir={}", tmpDir, e);
+      }
+
+    } catch (IOException e) {
+      log.error(
+          "mergeChunks failed: fileName={}, tmpDir={}, outputPath={}, lastChunkIndex={}",
+          fileName,
+          tmpDir,
+          outputPath,
+          chunkTotalIndex,
+          e);
+      throw e;
     }
 
-    // 병합후 임시 폴더 삭제
-    FIleChecker.deleteFolder(tmpDir);
+    log.info(
+        "mergeChunks done: fileName={}, outputPath={}, bytes={}, elapsedMs={}",
+        fileName,
+        outputPath,
+        totalBytes,
+        (System.currentTimeMillis() - start));
   }
 }