This paper describes an efficient mechanism for multicast tree reconstruction at source mobility in a source specific multicast (SSM) Source mobility makes its address changed so that the existing multicast tree corresponding to the home of address (HoA) of the mobile source should be rebuilt But this causes a large amount of packet delay during the reconstruction time This paper describes the reuse of the legacy multicast tree for a minimization of packet delay So we expect to improve quality of multicast service in mobile environment because of the reduction of the tree rebuilding time and the followed overhead.