A mobile ad-hoc network (MANET) is a characteristized by multi-hop wireless links, in the absence of any cellular infrastructure, as well as frequent host mobility. Existing on-demand multicasting protocols are categorized into tree-based and mesh-based strategies. In this paper, we propose a new on-demand multicast routing protocol, named spiral-fat-tree-based on-demand multicast protocol (SOM), for multi-hop mobile ad hoc wireless networks. We build a spiral-fat-tree as our multicasting tree. The main advantage of spiral-fat-tree is to keep the stability of multicast tree. Performance study shows that our proposed scheme outperforms the reactive tree-based and meshbased multicast protocols existed in the ad-hoc network.