The present invention is directed to a wireless communication system. More specifically, the present invention is directed to a method of transmitting an ACK/NACK (Acknowledgement/Negative-ACK) signal at a user equipment in the wireless communication system, the method includes receiving a first downlink control channel from a base station; receiving scheduling information through a second downlink control channel from the base station; deciding an uplink resource index for transmitting an ACK/NACK signal associated with the scheduling by considering the first control channel when the first control channel has a predetermined format; and transmitting the ACK/NACK signal to the base station by using an uplink resource indicated by the uplink resource index.