A method is provided for generating Acknowledgement/Negative Acknowledgement (ACK/NACK) information at a user equipment in a wireless communication system. The user equipment receives a plurality of codewords corresponding to data blocks through a plurality of downlink carriers from a base station, wherein each of the plurality of downlink carriers carries one or more codewords. The user equipment generates the ACK/NACK information by concatenating ACK/NACK bits corresponding to the plurality of codewords. When a corresponding downlink carrier carries two or more codewords, the ACK/NACK bits corresponding to the two or more codewords associated with the corresponding downlink carrier are concatenated in accordance with an index order of a corresponding data block. The ACK/NACK bits concatenated in accordance with the index order of the corresponding data block associated with each of the downlink carriers are concatenated again in accordance with an index order of a corresponding downlink carrier.