An Ad hoc network is the cooperative engagement of a collection of mobile nodes without the required intervention of any centralized access point or existing infrastructure, so they are vulnerable to many attacks and the security of the network can not be ensued. In this paper, we present a novel Intrusion Detection Mechanism based on the Trust Model (IDMTM) for mobile Ad hoc networks. In IDMTM, we employ two new concepts: “Evidence Chain (EC)” and “Trust Fluctuation (TF)” to accurately evaluate the trust value of a node in the network for judging whether it is malicious or not. Comparing with other Intrusion Detection System, IDMTM can greatly decrease the possibility of false-alarm with by efficiently utilizing the information collected from the local node and the neighboring nodes. Also, IDMTM can efficiently isolate internal malicious nodes from the networks and enhance the security without compromising the performance of the networks.