diff --git a/grader b/grader
--- a/grader
+++ b/grader
@@ -57,11 +57,14 @@
          the problem name must be specified by the next argument.
 
          additional options:
+            --all-sub    re-grade every submissions instead of just the latest submission of each user.
 
-            --all-sub    re-grade every submissions instead of just the latest submission of each user.
   sub:   re-grader the specified submission.
          The submission ID to be re-graded must be specified by the next argument.
 
+  options:
+    --err-log   log error to a file in the log dir
+
 (3) create stop-file to stop running grader in queue mode
 (4) You are here.
 USAGE
@@ -134,6 +137,8 @@
 
   options[:all_sub] = (ARGV.delete('--all-sub') != nil)
 
+  options[:err_log] = (ARGV.delete('--err-log') != nil)
+
   return options
 end
 
@@ -420,6 +425,13 @@
 
 #set loggin environment
 ENV['GRADER_LOGGING'] = log_file_name
+if options[:err_log]
+  err_file_name = log_file_name + '.err'
+  $stderr.reopen(err_file_name,"a")
+  log "STDERR log to file [#{err_file_name}]"
+  warn "start logging for grader PID #{Process.pid} on #{Time.now.in_time_zone}"
+end
+
 
 # register exit handler to report inactive, and terminated
 at_exit do