In the present work, an improved ejector theory, which was developed by Munday and Bagster is used, in order to study the thermodynamic behavior of the mixture NH3-H2O through an ejector. This ejector can be operated in combined ejector-absorption cycle using the binary mixture of ammonia-water. Kouremenos et al. showed that the heat gain factor (HGF) of a combined ejector-absorption system is 0.8-37.7% greater than those of the conventional absorption system. The design of ejector is based on Keenan et al.'s theory. Taking into account that the primary, secondary and back pressures are constant, there is a pressure before the shock where the flow entrainment ratio and the performance of the cycle (refrigeration or heat pump cycle) take maximum values. For this optimum value of flow entrainment ratio w, the value of area ratio A(t)/A(d) (cross section of minimum area of primary nozzle/cross section of constant area duct) can be estimated for an ejector's optimum design. The operation conditions were : primary pressure, 30-50 bar; secondary pressure, 3-5 bar; back pressure 10-14 bar; and mass fraction of ammonia vapor 97%.