Abstract: The present invention discloses a network address conversion system and the method thereof, which applies to products of VoIP and is compliant with Q.931 and H.245 protocols used by the videophone. The network address conversion system according to the present invention comprises a private network, a public network, a NAT router and a network server. The NAT router is used to convert the private IP address of the data packet from the private network into a public IP address for entering the public network, and to convert the public IP address of the data packet from the public network into a private IP address for entering the private network. The network server is connected to the public network to execute a public IP address query program, and then transmits the public IP address back to the node in the private network.