Capacity control of heat pumps and air-conditioners has traditionally been accomplished by cycling the compressor on and off. There is much energy lost and large temperature waving when cycling the compressor on and off. One obvious means of improving equipment efficiency and increasing reliability is to reduce the amount of compressor cycling. The improvement in efficiency and comfort can be realized through the use of a variable-speed compressor. When the load of air-conditioner varies, the variable-speed compressor is controlled to change the refrigerant flow by the inverter. The room temperature wave is lessened and the room can be heated or cooled quickly by running the compressor at high speed. The performance of variable speed air c0nditioner (VSAC) is analysed by a simulation tool. The cooling/heating capacity, the power input and the condensing temperature fall with the decreasing of the compressor frequency, but the evaporating temperature rises at the same time. The decreasing of the difference between condensing temperature and evaporating temperature benefits for the refrigerant cycle and the compressor operation. EER (Energy Efficiency ratio) rises with the decreasing of the compressor frequency, but when the frequency is too low, lower than 30Hz, the EER is also falls.