A method of performing hybrid automatic repeat request (HARQ) in a carrier aggregation system, the method includes receiving downlink data through a first subframe of a first carrier; transmitting acknowledgement/not-acknowledgement (ACK/NACK) for the downlink data through a second subframe of a second carrier; and receiving the downlink data again through a third subframe of the first carrier, wherein the first carrier is a carrier which s comprised of downlink subframe only.