Recently, some proposals suggest to use a peer-to-peer network instead of servers to facilitate SIP user registration and location in IMS communication systems. Due to the lack of central authority, security issue becomes one of the main problems in such P2P IMS systems. In this paper, we propose a novel security architecture for P2P IMS system. By using this architecture, our P2P IMS system becomes a fully distributed secure system in which there is not a central server indeed. Furthermore, our system is a scalable system which is capable of anonymous yet accountable real time communication based on SIP protocol which enables the system compatible with any existing IMS systems.