Hi ... thanx to the two experts above.
I slove my problem. This is how I slove it.
Redhat 9 mount my vfat hd by default( 755 ).
First, I add root to mygroup(gid=500). Then edit fstab like this:
/dev/hda7 /data vfat gid=500 0 0
This setting sets my vfat hd group as 500. But other user still can enter that /data folder since defualt is 755. So, I add umask
/dev/hda7 /data vfat gid=500,umask=007 0 0
I had test this setting, it works fine.
If the setting is unsecure, please let me know.
Thanx you guys help.