Disclosed herein is an air conditioner and control method of determining duration to be maintained at a reference temperature of a mode change based on an outdoor temperature. The air conditioner includes a compressor, an outdoor heat exchanger, an indoor heat exchanger, a four-way valve configured to guide refrigerant compressed in the compressor to either the outdoor heat exchanger or the indoor heat exchanger according to a cooling mode or a heating mode, and a controller configured to determine duration corresponding to a measured outdoor temperature, change the heating mode to the cooling mode when the measured indoor temperature is maintained at a first reference indoor temperature or greater for the determined duration, and change the cooling mode to the heating mode when a measured indoor temperature is maintained at a second reference indoor temperature or less for the determined duration.