The present invention relates to a wireless communication system. More specifically, the present invention relates to a method and a device for configuring TTI bundling at a UE with multiple carriers in the a wireless communication system, the method comprising: con-figuring a first cell with configured uplink belonging to a first base station (BS), wherein the first cell is configured with TTI (Transmission Time Interval) bundling enabled; receiving a message for configuring a second cell with configured uplink belonging to a second BS; and configuring the second cell in accordance with the message, keeping using TTI bundling for the first cell, if the first BS and the second BS are different.