A Voice over Internet Protocol (VoIP) service system using Network Address Translation (NAT) and a method of processing a packet in the system. The method includes the steps of: looking up a private address of a terminal located behind a NAT device in a previously generated NAT table using port information included in a packet transferred from a terminal located outside the NAT device to the terminal located behind the NAT device; and when a packet is transferred from the terminal located behind the NAT device to the terminal located outside the NAT device, generating an IP conntrack having the private address of the terminal located behind the NAT device as an originating address and having a public address of the terminal located outside the NAT device as a destination address.