Abstract: The wireless nature and inherent features of mobile ad hoc networks make them vulnerable to a wide variety of attacks. In an internal attack, the attacker gains normal access to the network and takes part in the network activities. The major challenges related to securing ad hoc networks are to make ad hoc network unconditionally secure against different external and internal attacks security solutions should be energy efficient to perform the functionality of the network (routing the packets properly and securely) if some of the nodes are captured or any node goes out of the radio range. To avoid the overhead of handling the network as a whole, the nodes are grouped into clusters. The challenges can be handled by formalizing the trust relationship between the nodes. To enhance security, it is important to evaluate the trustworthiness of nodes without depending on central authority. In this study, a cluster based trust mechanism is evaluated to mitigate the internal attacks. The entire network is divided into hierarchical group of clusters. Each cluster having a fully trusted Cluster Head (CH). Each node calculates the trust value for its one hop neighbours and sends it to CH. In turn, the CH issues the trust certificate to its member nodes if the trust value is positive otherwise that particular node is declared as misbehaving node. By simulation results it is shown that the cluster based trust mechanism provides better packet delivery ratio and resilience against internal attacks.