A method for transmitting ACK/NACK (Acknowledge/Negative ACK) information for two or more carriers at a user equipment in a wireless communication system which supports carrier aggregation. The method according to one embodiment includes generating an ACK/NACK payload including two or more ACK/NACK sets, wherein each ACK/NACK set is associated with a corresponding carrier and have one or more ACK/NACK bits; and transmitting the ACK/NACK payload through a PUCCH (Physical Uplink Control Channel) or PUSCH (Physical Uplink Shared Channel). A size of the ACK/NACK payload is given based on carrier configuration, the carrier configuration including a number of configured carriers and transmission modes for the configured carriers. The two or more ACK/NACK sets are concatenated in an order of carrier index.