A method for receiving Acknowledgement/Negative Acknowledgement (ACK/NACK) information at a base station in a wireless communication system. The method according to one embodiment includes transmitting a plurality of transport blocks through a plurality of downlink carriers to a user equipment. Each of the plurality of downlink carriers carries two or more transport blocks. The method according to the embodiment further includes, after transmitting the plurality of transport blocks, receiving the ACK/NACK information through one uplink carrier from the user equipment. The ACK/NACK information includes ACK/NACK bits corresponding to the plurality of transport blocks, and the ACK/NACK bits are concatenated in accordance with 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.