This paper discusses the use of Ejector as an expansion device as an energy efficient alternative to expansion valve in air conditioning system to improve the system performance. Expansion Process in Vapour Compression refrigeration system represents one of the important sources of irreversibility. Ejector was used instead of conventional expansion device which not only recover the expansion process loss by generating isentropic expansion process but also by increasing the cooling capacity and decreasing the power consumption. Initially experimental test was conducted on window air conditioner with standard expansion valve and then with ejector .Test were also conducted to re-circulate the liquid at the outlet of evaporator if any by using separator through valve to ejector. These act as secondary flow which was controlled by using hand valve. The coefficient of performance (COP) values for both conventional and modified ejector systems were experimentally investigated under same external conditions. Modified ejector refrigeration system shows 20% improvement in COP as compared with conventional system.