diff --git a/tools/releasetools/img_from_target_files b/tools/releasetools/img_from_target_files
index ebf67eb..7acc1fd 100755
--- a/tools/releasetools/img_from_target_files
+++ b/tools/releasetools/img_from_target_files
@@ -136,11 +136,7 @@
   # assert p.returncode == 0, "build system.img image failed"
 
   # TODO: Why the above common.Run() generate different system.img for crespo?
-  str_command = "mkyaffs2image -f ";
-  if OPTIONS.mkyaffs2_extra_flags is not None:
-    str_command = str_command + OPTIONS.mkyaffs2_extra_flags
-  str_command = str_command + " " + os.path.join(OPTIONS.input_tmp, "system") + " " + img.name
-
+  str_command = " ".join(build_command)
   print "running " + str_command
   exit_code = os.system(str_command)
   assert exit_code == 0, "build system.img image failed"