Ejector cooling systems are attractive due to their simplicity, reliability, low installation and operational cost.They can be driven by waste heat and solar energy. Ejector is the heart of an ejector system.The design of an ejector has been usually based on empirical equations derived from experimental results.Computational fluid dynamics (CFD) is a powerful tool to assist the ejector design. It may be used to simulate the effect of changing ejector geometry under different operating conditions.The ability to have visual display of the flow pattern makes it even a more attractive design tool. This paper presents a CFD study of changing ejector geometry including nozzle exit position, nozzle length, mixing section configuration and diffuser diameter. A base geometry for R123 as the working fluid was determined using empirical methods. The results show that the best primary nozzle exit position is around 5mm inside the mixing section. Diffuser diameter is the most critical for the ejector performance because a slight increase would result in a significant change in the entrainment ratio.The change of the nozzle exit diameter also has a large effect on the entrainment ratio.The mixing section entrance diameter has some effect, and the nozzle length has little effect. Curved nozzle entrance and mixing section entrance is a little better than the straight entrances.