Hi Chandrashekar ,
The portal activity report (PAR) feature enables portal administrators to gather and present information about portal activity, including information about the user's who logged on to the portal and about the content which is viewed.
Please see this link:
How to Configure Portal Activity Report - Portal - SCN Wiki
And this help link:
Portal Activity Report - Portal - SAP Library
There are series of steps you need to follow.
Hope this helps.
Regards