This paper presents the performances of new ozone-friendly refrigerant (R431a) in air-conditioning system were investigated experimentally and compared with R22 under transient conditions.R431a has no ozone depletion potential and very low greenhouse warming potential of less than 43 (ASHRAE Listed,2007).The variation in refrigeration rate of evaporator, work consumption of compressor and heat rejection rate of condenser with time from the start to stop of the air condition system have been examined. Test results showed that the refrigeration rate of evaporator and heat rejection rate of condenser drops almost parallel to each other, both with a decreasing rate with time. The power consumption on the other hand does not increasing much. The coefficient of performance of R431a decreased from rang 8.96 to 9.28% through (5000 sec) at different ambient temperatures. The refrigeration rate of evaporator decreased by 2.69% through 5000 sec at ambient temperature 30 C° for R431a .The heat rejection rate of condenser decreased by 2.34% through 5000 sec at ambient temperature 30 C° for R431a. Results showed that the coefficient of performance of R431a is 14.67 to 20.34% higher than that of R22. Compressor input power of R431a is 8.39 to 14.19% lower than that of R22 at different ambient temperatures.