I try to setup Samba in my Rehat Server 5, and shared root '/' to Windows XP.
The '/' can be accessed, no problem, all directories are ok, except 'var', no matter what, it says access denied. The directory just as same as other directory, nothing special.
Just wondering why, someone please help me out.

Probably it is a separate filesystem.

On your linux box type:
df -h /

Then type:
df -h /var

If both show only "/" in the "Mounted on" column /var is not a separate filesystem. However, if /var shows "/var" in the "Mounted on" column it IS a separate filesystem. You'll have to share it separately.

This is true also for NFS - when you export a filesystem only the files actually in that filesystem are exported. Any filesystem mounted on top of that filesystem is separate and has to be exported separately.

P.S. You really shouldn't export things like / and /var to other systems - these directories are for "system" files and sharing them out risks having someone inadvertently corrupt your system files which will crash your box.