A method and apparatus for establishing a session in a wireless communication system is provided. An entity of a primary radio access technology (RAT) system receives measurement results on the primary RAT system and a secondary RAT system from a multi-RAT device, determines a routing rule and a routing type for bearers which are appropriate for simultaneous transmission through the primary RAT system and the secondary RAT system, based on quality of service (QoS) and the measurement results, and transmits a routing control request message to a packet data network (PDN) gateway (P-GW). The routing control request message includes identifiers (IDs) of the bearers which are appropriate for simultaneous transmission, the routing type for each bearer, and the routing rule for each bearer.