With the development of network, multimedia will be the main application in next generation network. Voice is one of the most important applications. Recently a kind of P2P-based VoIP software, Skype, has been receiving more and more attention both in academia and industry. Skype claims that it's better than other VoIP software, because of its high call completion rate and superior sound quality. This paper will reveal Skype's technique and have a performance comparison between Skype and MSN Messenger, which uses traditional VoIP protocol. The result indicates that the voice quality of Skype is no better than traditional VoIP software and the great benefit of P2P involved with VoIP is that it can solve NAT and firewall problems.