Mobile Ad hoc Network (MANET) is a collection of more than two mobile nodes which can be self configured. It is an infrastructure-less network in which we do not need any kind of base stations or access points. It is dynamic in nature that is nodes are independent and can move in or out from the network any time or anywhere. Due to characteristics of MANET there are many security attacks possible on it. From various possible security attacks, we focused only on black hole attack. In this, malicious node pretends to be a good node and promotes as having shortest path to destination and once the path defines, it will start dropping packets coming to it. In this paper we detect and isolate both single and cooperative attacks. To improve MANET security against single and cooperative black hole attacks, a Security Algorithm Using Step Verification technique is used, in which we use Trust_Table to check the trust level of the nodes so that we can make a trustful node and a fake destination message is used to identify the trustful node.