A method for and apparatus for performing data transmission in a wireless communication system is provided. A wireless device acquires information including a threshold on an action, which is configured for a secondary radio access technologies (RAT) system, checks a condition to be triggered by the threshold and a measurement result of the secondary RAT system, performs a corresponding action according to the condition. The secondary RAT system is used for a User-Plane (U-plane) data, the threshold is changeable by at least one of a network preference including a primary RAT system or the secondary RAT system, a quality of service (QoS) information, and bearer information, the action includes whether an association/disassociation/data transition direction for the secondary RAT system is configured.