The present invention relates to a method and apparatus for transmitting an acknowledgement/not-acknowledgement (ACK/NACK) signal in a wireless communication system. The method includes the steps of: receiving a transmission block; and transmitting an ACK/NACK signal for the transmission block in a first subframe or a second subframe according to the size of the transmission block, wherein, when the size of the transmission block is smaller than or equal to a reference value, the ACK/NACK signal is transmitted in the first frame, when the size of the transmission block is greater than the reference value, the ACK/NACK signal is transmitted in the second subframe, and the second subframe is behind the first subframe in a time domain.