A VRF (variable refrigerant flow) air conditioning, having a number of air handling units connected to an external condensing unit, commonly uses two or more compressors in the system. The complicated piping system of the VRF air conditioning causes difficulties for refrigerant oils to return back to each compressor, especially for compressors operating in different frequencies. Compressors need lubrication in operation process, otherwise the compressor will burn out because of wearing and overheating. Therefore, whether refrigerant oils can successfully return back to each compressor has become a key issue in the VRF air conditioning system. To this regard, this report studied refrigerant oil retention of each compressor operating in different frequencies in an air conditioning system with two compressors in parallel and proposed a simple strategy for the refrigerant oil retention of compressors to keep lubricants balanced in two compressors.