An active handoff system and method of a dual mode terminal capable of connecting to a wireless network, such as wireless local area network (LAN), and a mobile communication system are provided. The handoff method includes the steps of assigning, at a packet data service node of the mobile communication system, an IP address to the dual mode terminal connected to the service network, establishing, at the packet data service node, a tunneling path to the wireless LAN when the dual mode terminal moves from the mobile communication network to the wireless LAN, and disconnecting, at the packet data service node, the tunneling path to the wireless LAN, and establishing a tunneling path to another packet data service node whose Internet Protocol (IP) address has been established by the packet data service node, when the dual mode terminal moves from the wireless LAN to the mobile communication system. A dual mode terminal that moves from a 3G mobile communication system to a wireless network in an active state maintains an existing IP address assigned from an active 3G mobile communication system so that the dual mode terminal can maintain an upper layer session and transmit an IP packet to a terminal located in a 3G mobile communication system.