Mobile ad hoc network is having dynamic infrastructure in which mobile nodes move around freely and communicates by forwarding and receiving packets without any support of pre-existing infrastructure. Thus the mobile ad hoc network is highly vulnerable to various kinds of attacks and suffers for lack of security in real-time applications. This paper discusses one of the most vulnerable behaviors known as a black hole attack. Black hole node advertises itself as having valid route and drops packets maliciously. In this paper, the authors have identified the security of MANET uses AODV protocol by analyzing the impact of the black hole attack on it. The simulation environment is implemented by NS-2, network simulator.