A method of performing a hybrid automatic repeat request (HARQ) of a relay station in a backhaul link of a multi-carrier system is provided. The method includes: receiving backhaul downlink data through at least one carrier between a first carrier and a second carrier; transmitting acknowledgement/not-acknowledgement (ACK/NACK) for the backhaul downlink data through an uplink component carrier; and receiving new backhaul downlink data or retransmitted backhaul downlink data in accordance with the transmitted ACK/NACK through at least one carrier between the first carrier and the second carrier, wherein the first carrier is a component carrier dedicated to the backhaul downlink between a base station and the relay station, and the second carrier is a component carrier used in both the backhaul downlink and an access downlink between the relay station and a relay user equipment at different times.