The R134a refrigerant which is HCFC refrigerant as the substitute refrigerant of CFC refrigerant was used in the automobile air-conditioning system. The ODP(Ozone Depletion Potential) of R134a refrigerant is zero, but GWP(Global Warming Potential) is very high as 1300 and the energy performance of R134a refrigerant is excellent than that of R1234yf refrigerant. In this study, the performance of automobile refrigeration system with the mixing ratio of R134a and R1234yf was investigated and compared according to outdoor air temperature. As a result, the cooling capacity of the automobile refrigeration system with an increasing of mixing rate of R1234yf was gradually decreased. The COP of automobile refrigeration system was decreased proportionally with an increasing of mixing rate of R1234yf in R134a system.