A network assignment method of a gateway in a wireless communication system including a first communication network and a second communication network is provided. The method includes receiving, when a terminal is connected to the first communication network, a create session request message from a second mobility management entity associated with a second base station operating the second communication network, allocating second attach information for establishing a bearer between the gateway and the second base station based on first attach information generated for connection to the first communication network, and transmitting a create session response message including the second attach information to the second mobility management entity in response to the create session request message.