A device and method for handling HARQ feedback in a mobile communication system are disclosed. The HARQ feedback handling method includes: analyzing a control message from a base station to recognize presence of HARQ feedback relationships between downlink carriers and uplink carriers; determining an uplink carrier (a downlink carrier) to support HARQ feedback in response to downlink traffic (uplink traffic) sent through a downlink carrier (an uplink carrier); and sending (receiving) HARQ feedback through the determined uplink carrier (downlink carrier).