The effects of an internal heat exchanger in a refrigerant system with carbon dioxide on its cooling capacity and cooling coefficient of performance were numerically investigated. First, experiments were carried out for a refrigerant system with carbon dioxide to obtain volumetric and isoentropic efficiency and to verify numerical accuracy. Second, the effects of varying internal heat exchanger area, environmental temperature, lower side pressure and higher side pressure on cooling capacity and cooling coefficient of performance were numerically investigated. By comparing calculated results with experimental ones. our Simulation predicts capacity in the evaporator with +5.1% error, cooling coefficient of performance with +5.2% error, and capacity in the gas cooler with +3.0% error. When the environmental temperature is high and superheat is equal to 0 due to low performance of the evaporator, an internal heat exchanger gives a larger cooling capacity and cooling coefficient of performance. In particular, when the gas cooler exit temperature is close to the critical temperature, the internal heat exchanger gives higher performance.