A fuzzy defrost control algorithm for the multi-type heat pump system was developed. In the fuzzy defrost control algorithm, the air temperature difference at the outdoor unit and the refrigerant pressure difference at the compressor were used as input variables, and the defrost starting time and the defrost time interval were used as output variables. This fuzzy algorithm was applied to the multi-type heat pump system and tested in the five dynamic environmental chambers. Test results show that the newly developed control algorithm is more effective than the conventional control algorithm in the removal of frost formed at the outdoor unit of the heat pump.