The performance of a new refrigerant, R1234yf/R134a (with a mass ratio of 89∶11), was studied as alternative to R134a in automotive air conditioning systems using micro-channel heat exchangers. The results show that the cooling capacity of R1234yf/R134a and R134a is similar. The COP of R1234yf/R134a is lower than that of R134a by 4%9%. The average compressor discharge temperature of R1234yf/R134a is up to 10℃ lower than that of R134a. By adding 11% (mass) R134a, R1234yf/R134a can be made non-flammable. In addition, R1234yf/R134a has no ozone depletion potential, with a global warming potential of less than 150, thus meeting the requirement of European mobile air-conditioner directives. R1234yf/R134a can be used as an environment-friendly replacement for R134a in automotive air conditioning systems, with minor modifications.