Is there any link to download ab Apache benchmark

On Ubuntu, I can install ab without installing all of Apache via the apache2-utils package. So:

sudo apt-get install apache2-utils

Just download Apache (www.apache.org). It comes with it (in ApacheX.X/bin)

...Guessing (from one of your other questions) that you're using a Mac... there appear to be instructions here:
http://switch.richard5.net/isp-in-a-box-v2/installing-apache-on-mac-os-x/

(if not, I can probably help with a Windows installation, but in general, Google is your friend!)

A list of mirrors for the windows binaries can be found here.

There are some basic instructions here:
http://www.ricocheting.com/how-to-install-on-windows/apache

...basically, install it, and the ab.exe will be in the 'bin' subdirectory of the installation