Steam jet ejectors are the essential part in refrigeration and air conditioning, desalination, petroleum refining, petrochemical and chemical industries. A greater understanding of flow physic inside an ejector plays an important role in its performance improvement. In this study, analytical algorithm is developed for design of steam ejectors. The algorithm gives the flow ratio (motive to suction flow rate) as a function of the expansion ratio and the pressures of the entrained vapor, motive steam and compressed vapor. The compression ratio and back pressure variations were studied in ejector flow ratio with expansion ratio of 5 and 50. It showed that compression ratio increase by increasing the flow ratio. Also in a similar flow rate, compression ratio for ejector with expansion ratio of 50 is greater than compression ratio in the ejector with expansion ratio of 50, due to more vacuum in the case with expansion ratio 50. Then, the code results were compared with experimental results that showed appreciate agreement with other results. Finally, Mach number variations from nozzle exit to discharge diffuser were obtained by code. Results showed that the pressurized condition causes the lowering of expansion angle, thus resulting in smaller jet core and larger effective area. The expanded wave is further accelerated at a lower Mach number. Therefore, the momentum of the jet core is reduced. However, the enlarged effective area allows a larger amount of secondary fluid to be entrained.