thank you Hacker.
I was trying to create RHEL5 boot.iso from RHEL6-boot.iso. I have downloaded RHEL6-boot.iso and then tried created the boot.iso for RHEL5.Also I have cutomized the isolinux.cfg as based on my requirements. But when I tried to boot, it was not showing me the customized labels which I did.
I suspect vesamenu.c32 is not working properly. Is there any way to make it work using RHEL6-boot.iso ?