Security related issues are of serious concern in MANET (Mobile Ad hoc NETwork). Shared wireless medium and lack of central administration makes MANET more vulnerable to security threats. An intruder passes an intermediate node into the MANET and introduces several kinds of attacks on the data transfer occurring between nodes. If the MANET can detect those attackers as soon as they enter the network, then the consequences caused by those compromised nodes can be minimized. In this paper, the authors consider black hole attack in mobile ad hoc network, where all data packets are absorbed by the malicious nodes.