A method for generating Acknowledgement/Negative Acknowledgement (ACK/NACK) information at a user equipment in a wireless communication system. The method according to one embodiment includes receiving a plurality of transport blocks through a plurality of downlink carriers from a base station. Each of the plurality of downlink carriers carries two or more transport blocks. The method according to the embodiment further includes generating the ACK/NACK information by concatenating ACK/NACK bits corresponding to the plurality of transport blocks. The ACK/NACK bits are concatenated using an index order of the downlink carriers and an index order of the two or more transport blocks associated with each of the downlink carriers. The index order of the downlink carriers is used firstly.