Abstract: In Ad hoc networks, low reliability of wireless link makes the scalability of multicast group degrade drastically, and FEC(Forward Error Correction) technique and NACK(Negative ACKnowledgement)-based mechanism, which guarantees reliable transport of multicast in wired networks, is not adapt to be deployed in Ad hoc networks. In this paper, focusing on improving the scalability of multicast in Ad hoc networks, a Reliable Multicast Transport Scheme (RMTS) consist of PDA(Packet Dispersal Algorithm) and ARM(Active Retransmission Model) is proposed to guarantee reliable transports of multicast. Both theory analyses and simulation result show the merits of validity, simpleness, low resources-occupied and moderate overhead.