Disclosed are a method and an apparatus for transmitting reception acknowledgement for an HARQ (hybrid automatic repeat request) in a wireless communication system. A terminal receives the information on downlink resource allocation through at least one downlink carrier among plural downlink carriers. In addition, the terminal receives a downlink transmission block through a downlink shared channel indicated by the downlink resource allocation. The terminal transmits an ACK/NACK signal for the downlink transmission block through an uplink control channel. If at least one downlink carrier is a primary carrier, the uplink control channel uses a primary resource. Otherwise, the uplink control channel uses a secondary resource.