1.In the internal disk is centos 5.0,
working ok (if only this disk, the kernel will show in GRUB menu, which you can choose to boot).
2.And then install redhat enterprise 5.0 on a external disk,
...choose a option (use GRUB and install MBR on sda1(the internal disk))...
3.Now when boot in to the GRUB menu, only can see the redhat kernel in the list, not centos. Use the internal disk is no problem , since I can mount it,but I just want it show in the GRUB menu...

Any one can give me a clue, what may cause this problem.

Thx in advance.

From what I understand, you did overrode the CentOS' grub configuration at the MBR by installing red hat's grub

You could however try:

- boot into red hat, add the centos kernel line in the /boot/grub/menu.lst

If you're unsure about what to add, mount the centos partition, say /mnt/centos, and check the file /mnt/centos/boot/grub/menu.lst

You could also chroot to the centos partition and reinstall centos' grub but that will override the current so red hat's kernel line will be lost and you'll need to add it manually later.