An air conditioner including an outside unit, which includes a compressor and an evaporator, an outside temperature sensor attached to the outside unit to sense outside temperature, an outside humidity recognition part attached to the outside unit to recognize information about outside humidity, a low pressure sensor that senses an evaporation pressure of the evaporator, and a control part that controls an operation of the compressor, based on both information about dew-point temperature sensed from the outside temperature sensor and the outside humidity recognition part and information about the evaporation pressure sensed from the low pressure sensor. The control part changes an operation frequency of the compressor according to whether the evaporation pressure is not lower than a preset reference low pressure, to prevent frosting of the evaporator.