Peer-to-Peer (P2P) worms have become a serious problem on the Internet because of its adaptive propagation features. Due to the complexity of the problem, no existing work has solved the problem of modeling the propagation of P2P worms, especially when quarantine of peers is enforced. This paper presents a study on modeling the propagation of P2P worms under quarantine. We strive to find a quarantine tactic which is able to protect the majority of the peers by quarantining only the minority of them. Our major contributions in this paper are firstly, we propose a novel logic matrix approach to modeling the propagation of P2P worms; and secondly, we find the impacts of the two different quarantine tactics on the propagationcharacteristics of P2P worms. Motivated by our aspiration toinvent an easy-to-employ instrument for worm propagation research, the proposed approach models the propagation processes of P2P worms by difference equations of logic matrix, which are essentially discrete-time deterministic propagation models of P2P worms. To the best of our knowledge, we are the first using logic matrix in network security research in general and worm propagation modeling in particular.