In this decade, peer-to-peer (P2P) applications have emerged as popular tools for sharing data and they have gained much attention. Such P2P applications build overlay networks on top of existing infrastructure networks and disseminate data in a peer-to-peer manner. How well the P2P overlay network maps onto the underlying physical network topology can significantly impact P2P application performances. In the literature, little work has quantitatively studied the effects of underlying physical network topologies on P2P application performance. In this paper, we used simulations to evaluate the effects of underlying physical network topologies on the performance of the real-life BitTorrent protocol and application. Our results show that underlying physical network topologies significantly affect P2P application performances.