Vulnerability of a Network (can be an office LAN or computer systems connected together for secure data communication) is defined as susceptibility of a network for a successful intrusion or attack. Network's vulnerability depends on some specific factors. In this paper we have used Rough Set theory to trim down the massive data of factors. The paper considers various possibilities with different possible combinations of attack factors and deducting rules, based on Rough set theory. Thus, determining Vulnerability factor of the Network; describing proneness of the network for a successful attack. Albeit the paper considers only few factors, there can be enormous number of factors. © 2013 IEEE.