The present invention provides an NAT (Network Address Translator) traversal method in Session Initiation Protocol (SIP) for solving the problems of SIP in Internet phone (VoIP) under current Internet environment. In other words, the present invention solves the SIP problems caused by NAT (Network Address Translator) that P2P (Peer to Peer) transmission cannot traverse the NAT firewall directly. The major content of the present invention is that the computer conducts multiple registrations before issuing an Invite message in order to detect the rule of the NAT server to assign port number and IP address. Additional commands or parameters beyond the SIP standard are not required for the NAT traversal function. Hence, the capability of SIP for a NAT traversal function is provided without no additional commands or parameters.