An air conditioner, having an outdoor unit and an indoor unit, to perform a heating operation and a defrosting operation, the air conditioner including a detection unit to detect a state of at least one selected between the outdoor unit and the indoor unit and to output the detected value, a controller to determine whether the air conditioner is in a stable state when the defrosting operation is completed and, upon determining that the air conditioner is in the stable state, to control the detected value output from the detection unit to determine entry time of the next defrosting operation, and a storage unit to store a value detected in the stable state. The entry time of the defrosting operation is accurately determined, thereby minimizing the number of times of the defrosting operation during the heating operation.