The present invention relates to an internet protocol (IP) mobile communication system and a session initiation method thereof. In the IP mobile communication system, a session initiation process using a session initiation protocol is performed for real-time mobile multimedia communication. At this time, not only multimedia codec but also session and quality of service (QoS) information is negotiated from a transmitting terminal to a receiving terminal, and is negotiated between relating network elements. An access manager controls a mobile access router which is a gateway of an access network according to the negotiated session and QoS information. Therefore an additional QoS negotiation process is not necessary for a session guaranteeing the QoS between a terminal and the mobile access router, between the mobile access routers, and between the mobile access router and the receiving terminal.