Abstract: The flow field in steam ejector was mathematically modeled and numerically simulated with Fluent software.The influence of the mixing chamber geometry,including its axial length,inlet and throat diameter-ratios,on entrainment ratio was investigated.The simulated results show that the mixing chamber structure has a major impact.Roughly speaking,the entrainment ratio changes in an increase-decrease manner with an increase of either the axial length,or of the inlet diameter ratio,or of the throat diameter-ratio.A throat diameter-ratio,either too large or too small,severely deteriorates the performance of steam ejector.The optimized dimension of mixing chamber includes: a tapered section length of 50 mm,a constant diameter section length of 10 mm,a throat diameter-ratio of 5.63 and an inlet diameter-ratio of 2.22.We suggest that the simulated results be of some technological interest in design of steam ejectors.