1) I am using redhat advance server 4
2) want to recover my root password
3) I boot the box with as4 boot cd using linux rescue at prompt
4) then I mount the system with read-write mode
5) when I try to change the password using passwd root command it gave the following error

unable to starr pam

You missed an important step. After mounting the drive, cd to the drive (what would normally be root when booting from the drive), and type "chroot ." You should now be in / of your installation, and can type "passwd" to change the root passwd.

Also, depending on your installation, you can boot into a failsafe sudo rescue mode without using the cd. Just add "single" to the boot command (in lilo, type "linux single", in grub, hit <esc> to get to the boot menu, then "e" to edit the boot menu, <enter> to save the change (temporary, not permanent), and b to boot.

still it gave the following error

error :-.................

passwd: user_u:system_r:unconfined_t is not authorized to change the password of roo

The other option is to reboot in rescue mode, mount your root directory, and edit /etc/passwd. Change the follwoing line as follows:
root:$1$dL5vHy7c$R/1.GXWRTErRescKNlmQW1:0:0:root:/root:/bin/bash <--original (from my system - yours may vary

root::0:0:root:/root:/bin/bash <--New

If it originally looks like this "root:x:0:0:root:/root:/bin/bash" then you have password shadowing enabled (usually default). In that case, edit the /etc/shadow file the same way.

Also, if these methods don't resolve the issue, you can try to boot into single user mode (as described in my earlier post). If it also asks for a root password, you can disable that by editing /etc/inittab:

# What to do in single-user mode.
~:S:wait:/sbin/sulogin <-- change to /sbin/sh

It work after I remount it ,

mount -o remount,rw <device-name>

anyway Thanks man.