Because of global warming reality, CO2 (R-744) can be used an alternative refrigerant for mobile air-conditioning systems. In this study, the coefficient of cooling performance (COP) and energy requirement for operating of a mobile CO2 air-conditioning system compressor were investigated. In the analysis, various gas cooler pressure and outlet temperature from the gas cooler of the refrigerant were used as variable parameters. The evaporation temperature of CO2 and superheating value of CO2 in the internal heat exchanger were selected as + 5 degrees C and 10 degrees C respectively. The compressor isentropic efficiency was used as 0.7. The pressure losses in the system components were neglected. A computer program was used to obtain solutions of the equations and properties of CO2. It was concluded that, the COP values of the system reach a maximum value with variation of the gas cooler pressure and outlet temperature from the gas cooler of the refrigerant. The COP value variation speed with outlet temperature from the gas cooler of the refrigerant slowed down with the increased gas cooler pressure. The results were presented with diagrams.