A method for supporting backward compatibility of Multimedia Broadcast and Multicast Service (hereinafter referred to as MBMS) in a mobile communication system of Wideband Code Division Multiple Access (hereinafter referred to as WCDMA) is disclosed. An example method recites mapping relationship between the MBMS service identifier and the RAB identifier and judging the capability of the RNC by the SGSN, and accordingly, determining if the MBMS service is backwards compatible and if user equipment is compatible with different versions.