Disclosed is a method for operating an air conditioner in a warming mode, including the 100% operation performing step of operating/stopping all of compressors, the load determining step of determining a warming load to be eliminated after completion of the 100% operation performing step, the 100%/X% operation performing step of operating all of the compressors when it is determined at the load determining step that the warming load is not large, subsequently stopping a part of the compressors, and subsequently stopping the remainder of the compressors, the sensing step of sensing the number of stoppages of the remainder of the compressors after completion of the 100%/X% operation performing step, and the X% operation performing step of operating/stopping the remainder of the compressors when the sensed number of stoppage times reaches a predetermined number. Since the 100%/X% operation is carried out in response to a warming load generated after the 100% operation, this method properly copes with the warming load at an initial stage of the warming mode. Further, since the X% operation is carried out after the 100%/X% operation is repeated in a predetermined number, the method of the present invention precisely and promptly copes with the variation of the warming load.