An ad hoc network is a collection of mobile nodes that dynamically form a temporary network. It operates without the use of existing infrastructure. One of the principal routing protocols used in ad-hoc networks is AODV (Ad-hoc On demand Distance Vector) protocol. The security of the AODV protocol is compromised by a particular type of attack called 'Black Hole' attack. This paper analyze the black hole attack which is occurs in ad hoc network. In this attack a malicious node advertises itself as having the shortest path to the node whose packets it wants to intercept.