In this paper, we have designed and implemented the system architecture for extensible mobile instant messaging and presence service over the IMS. More specifically, we have implemented the SIP proxy servers for IMS P-CSCF and S-CSCF for the realization of extensible mobile instant messaging and presence service. The salient feature of the proposed system architecture is that the architecture is based on IETF SIMPLE standard, and the extensibility of the presence service is achieved by implementing theOMA IMPS specification. An efficient algorithm is presented for achieving the extensible presence service over IMS. The implementation results show that the proposed system architecture can effectively provide not only interoperability but also extensibility for representing userspsila status information in the 3 G mobile communication environment.