The present study used computational fluid dynamics (CFD) technique to examine the impacts of the nozzle exit position (NXP), the area ratio of the constant area zone to the primary nozzle throat, and the mixing chamber diameter on the ratio of entrainment and also the performance of the ejector. The effect of the parameters is explained by using different geometric structures under the same boundary conditions. The simulation validated by using the dissemination of static pressure and the given experimental data of entrainment ratio which is available in the literature. It is found that, the simulation error between the experimental data and the (CFD) results of entrainment ratio of the ejector are 10.4% and 1.5% for pressures of 143.3 kPa and 270 kPa, separately. The results show that the steam ejector performance is sensitively affected by increasing the entrainment ratio. Keywords: Ejector; Entrainment ratio; area ratio; NXP; mixing chamber diameter.