Because of the global warming, usage of natural refrigerants is being common in large scale. CO, cooling systems especially preferred at vehicle air conditioners and hot water heat pump applications. In this study, a single stage heat pump using CO2 refrigerant and having transcritical cycle is theoretically modeled. The effect of gas cooler outlet temperature for exergetic efficiency of heat pump is presented. Range of evaporating temperature is selected between -10 and +10 C degrees values. Calculations are made by software (EES-Engineering Equation Solver), and results are presented graphically.