Based on a three-dimensional CFD code developed in Fluent ANSYS environment, a sample geometry of the R744 vapor ejector was numerically analyzed. The energy performance, represented by the ejector efficiency, was simulated for three distinctive sets of boundary conditions. Also, to investigate the local irreversibility effects inside the ejector passages, one-dimensional profiles of pressure, the Mach number, and the cross-section averaged specific entropy were analyzed. For all the cases a relatively low percentage of the total irreversibility is found to occur in the motive and suction nozzles, i.e., less than 15% in total for both nozzles. On the other hand, high losses were registered for mixer and diffuser, up to 49% and 63% of the total irreversibility, accordingly. The CFD simulations were validated against the results of laboratory experiments, finding some discrepancy between the measured and entrained mass flow rates. It was concluded that these discrepancy could arise from the combination between the model assumptions and boundary values.