The absence of an online trusted authority makes the issue of key revocation in mobile ad hoc networks (MANETs) particularly challenging. In this paper, we present a novel self-organized key revocation scheme based on the Dirichlet multinomial model and identity-based cryptography (IBC). Our key revocation scheme offers a theoretically sound basis for a node in MANETs to predict the behavior of other nodes based on its own observations and reports from peers. In our scheme, each node keeps track of three categories of behavior defined and classified by an external trusted authority, and updates its knowledge about other nodespsila behavior with 3-dimension Dirichlet distribution. Differentiating between suspicious behavior and malicious behavior enables nodes to make multilevel response by either revoking keys of malicious nodes or ceasing the communication with suspicious nodes for some time to gather more information for making further decision. Furthermore, we also analyze the attack-resistant properties of our key revocation scheme through extensive simulations in the presence of adversaries.