The present invention is directed to a wireless communication system. Particularly, the present invention is directed to a method and an apparatus of transmitting an ACK/NACK (acknowledgement/negative ACK) signal by a user equipment in a wireless communication system, which includes receiving a plurality of data units from a base station, bundling ACK/NACK results for a plurality of the data units per time or frequency interval to obtain at least one bundled ACK/NACK result, performing ACK/NACK multiplexing over the at least one ACK/NACK result to determine an uplink resource for ACK/NACK transmission and a bit sequence, and transmitting the bit sequence to the base station via the uplink resource for the ACK/NACK transmission.