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Without requiring multicast support from the underlying networks, overlay multicast has the advantage of implementing inter-domain multimedia multicast communications. Usually, overlay multicast protocols employ two different topologies: r-ary tree and m-D mesh. In this paper, we study the influence of topology selection on multimedia multicast performance. We present a set of theoretical results on the worst performance, the average performance, and the performance difference along the link stress, the number of overlay hops, and the number of shortest paths for r-ary tree-based and m-d mesh-based multicast, respectively. Furthermore, through simulations in NS2, we observe and compare tree and mesh topologies along the metrics analyzed theoretically. Simulation results match our theoretical analyses. Finally we give our evaluations of and insights into these two kinds of multicast when used to transmit multimedia streams. The selection of overlay topology is application dependent. To the best of our knowledge, this is the first evaluation of multimedia multicast performances in different overlay topologies. We believe that this study is useful for protocol design of target multimedia applications and for investigating multicast functions.