A method and apparatus for applying assistance information for traffic steering between a 3rd generation partnership project (3GPP) access network and a non-3GPP access network in a wireless communication system is provided. A user equipment (UE) receives assistance information for traffic steering through a dedicated signaling from an eNodeB (eNB), and starts a timer upon entering an idle mode. The UE applies the assistance information received through the dedicated signaling until the timer expires. After the timer expires, the UE applies assistance information received through a broadcast signaling.