diff --git a/weed/s3api/filer_multipart.go b/weed/s3api/filer_multipart.go
index 414ba4bb2..3cf0971f6 100644
--- a/weed/s3api/filer_multipart.go
+++ b/weed/s3api/filer_multipart.go
@@ -122,7 +122,7 @@ func (s3a *S3ApiServer) completeMultipartUpload(input *s3.CompleteMultipartUploa
 	}
 
 	entryName := filepath.Base(*input.Key)
-	dirName := filepath.Dir(*input.Key)
+	dirName := filepath.ToSlash(filepath.Dir(*input.Key))
 	if dirName == "." {
 		dirName = ""
 	}