In an ad hoc network, each mobile node plays the role of a router and relays packets to final destinations. The network topology of an ad hoc network changes frequently and unpredictable, so that the routing and multicast become extremely challenging. In this paper, we describe the multicast routing representation using fuzzy Petri net model with the concept of immediately reachable set in wireless Ad Hoc networks which all nodes equipped with GPS unit. It allows structured representation of network topology, and has a Fuzzy Reasoning Algorithm for finding multicast tree and improves the efficiency of the ad hoc network routing scheme. Therefore when a packet is to be multicast to a group by a multicast source, a heuristic algorithm is used to compute the multicast tree based on the local network topology with a multicast source. Finally, the simulation shows that the percentage of the improvement is more than 15% when compared the IRS method with the original method.