Abstract: Based on the construction of an ejector, a 2-D CFD model of the steam ejector was developed to investigate the influence of its operating parameters including operating, driving and mixed fluid pressures and constructional parameters including the length of contraction section of mixing chamber, length and diameter of second throat, length of diffusion chamber and construction of primary nozzle on the performance of a steam ejector. Simulation results indicated that the optimal or maximum jet coefficient depends on the length and diameter of second throat, diameter of primary nozzle throat and the area ratio of primary throat to nozzle throat (φ). CFD is thus proved a powerful tool to predict the properties of ejector and, what's more, it can be used to make sure of the flowing/mixing process of the fluid in ejector.