My geuss is that either your system doesn't have enough memory or your
swap drive is to small.

you're computer has too
little total memory. Apache is really rock solid, and you probably are
running too much. Get some more ram, or make a bigger swap file...

my machine has 512 mb memory ve 512 mb swap files it runs an apache
server postfix smtp I have nearly 50 user for web and 300 email users
some times I see this msg Out of memory killed httpd and my machine
stops I can'T solve the problem

I can't imagine you using all that memory with the given
users, so it's obviously a bug. What version of apache are you running?
You might try switching versions, I still haven't tried Apache 2.0,
but you might do that too.

I thought if you were running 512MB of ram you didn't
need a swap file???

Technically you don't. However, you would need to have one set up in memory, instead. This can be more than many users are willing to put up with.
Also it is not a swap file. It is a swap partition which Linux usually sets up automatically.

Check out your /etc/fstab file. Look for a line that says swap.

In mine it looks like this:

/dev/hdd1 swap swap defaults 0 0

If you see something similar then you do indeed have a swap partition.