This paper proposes a new mechanism for Session Initiation Protocol (SIP) over Mobile IPv6. In this mechanism, a home agent (HA) on home subnet acts as a redirect server and a registrar for SIP as well as a home router for IPv6. Thus, a binding cache in the HA contains location information for SIP as well as home registration entries for IPv6. An access router on foreign subnet acts as only a router that offers a domain name. To implement the proposed mechanism, some messages used in network layer are newly defined, such as a router advertisement, a router solicitation and a binding update. In the proposed mechanism, a mobile node doesn’t require dynamic host configuration protocol (DHCP) and thus both home and foreign subnets don’t need DHCP servers, unlike existing mechanisms on IPv4. Analytic performance evaluation and comparison are made, which shows the proposed mechanism is more efficient in terms of delay than existing mechanisms.