Steam ejectors can have various kinds of performance characteristics, but when high vacuum for mixing chamber or high compression ratio for entrained fluid is required for their performances, as is usual with such a case, the mixture of driving steam and entrained fluid should be kept in supersonic and fulfilled flow state at the diffuser throat inlet. The characteristics of steam ejectors in this flow state are called high vacuum characteristics.In this flow state, the entrainment performance of an ejector, as expressed by the relation between the pressure of mixing chamber and the entrained flow rate, is dctermined independently of the back pressure of the diffuser. Therefore, a method of calculating the entrainment performance has been proposed, taking into consideration the drag force of steam jet from the nozzle and the entrained fluid passage area as measured cross-sectionally at the convergent part of the diffuser.This desirable flow state depends largely on the outside conditions of the ejector. When the back pressure of the diffuser is raised above the limit value-the critical back pressure-, the high vacuum characteristics cannot be observed and the vacuum of mixing chamber will suddenly decrease. For the compression performance of the diffuser, another calculation method has been proposed for the purpose of obtaining the critical back pressure, taking into consideration the mixing of driving steam and entrained fluid with the mean velocities of the mixture relative to flow rate, momentum and energy, though, in the previous calculation methods the mixing process was neglected.From the calculation method presented in this paper, Eq. (24) has been obtained for the entrainment ratio which is the ratio of the flow rate of entrained fluid to that of fluid taken as a basis or at a base state. Comparison of Eq. (24) with the experimental results by Holton and Schulz3, 4) has proved that the drag coefficient of steam jet χ' is independent of the molecular weight of entrained fluid, but has a tendency to increase with the temperature rise of entrained fluid. If the drag coefficient χ' and χ are kept constant, the following formula may hold which represents the fact that the flow rate of entrained fluid will be proportional to the square of dimension ratio, when ejectors are geometrically similar.