I have 2.6.18-8el5 kernel and I wants to update the kernel to PAE.
From where I will get the PAE kernel ?

It should be part of the standard repos (If you need pae and the machine supports it you should really be running 64b unless there is some overriding reason not to.)

Edit: To clarify-- I mean if you NEED PAE (esp big mem) you SHOULD BE running a 64b kernel instead unless you have an overriding reason to not be or your hardware isn't 64b.

There is no PAE kernel for x86_64 (64-bit) version. Standard kernel on x86_64 is already PAE (capable).

@bhageshp, you are posting RHEL/CentOS issue in Fedora forum. I suggest Admins move it to Red Hat forum.

Moved: This thread is more suitable in Red Hat and has been moved accordingly to help your thread/question get the exposure it deserves.

What distribution?

IIUC, you have "Ubuntu" listed as your distribution in the section to the left of your post. You posted originally in Fedora. You posted a version number that should be either Red Hat or Centos.

It's always a good idea to tell us the distribution rather than let us figure it out.

And in el5 Centos (? or Red Hat), I think the package is named kernel-PAE so in Centos
As in other threads, I disagree.

Upgrading to PAE is easy. Switching to 64 bit mode is hard. That may be a good enough reason.

For 4GB to 8GB (or maybe even more), PAE might be perfectly OK and 64bit might have no advantages. It depends a lot on how you use the system.

I've not had any issues switching from 32->64b in Linux in the past year or more. When 64b was first coming out yes, there were definitely issues, now? not so much unless you're using a bunch of 32b apps that are in house... which most aren't, they're using stuff right outta the repos. There are definitely instances where I wouldn't suggest 64b but for 95% of people there are no significant issues anymore.

I don't think I even implied that one should expect problems with 64 bit. Most of the several Linux systems I use are 64 bit. I do not recommend against 64 bit.

My point was that the same is true of PAE. (The problems are ancient history).

The OP asked for PAE. When someone asks for PAE and you have good reason to believe it is terrible idea, it is appropriate to tell them to use 64 bit instead of telling them how to get PAE.

But when PAE is reasonable, we don't need to know whether 64 bit would be slightly better or slightly worse or whether this is some uncommon case in which 64 bit is unreasonable. Assume the OP is competent to make that decision.

I said
Maybe that was misleading. I didn't mean it would be hard to make 64 bit work after you switch to it. I would expect no problems. I meant that Linux is already installed. Installing a kernel package with the package manager followed by one reboot is easy. Reinstalling Linux is harder and depending on disk partitioning, backing things up before the reinstall may be much harder. Switching to 64 bit requires reinstalling Linux.

One of the reasons for switch to 64-bit in RHEL/CentOS is that KVM virtualization is only present (or updated) on 64-bit systems.

I don't make that assumption on a forum dedicated to helping newbies, especially when the question is relatively that simple.