Abstract: This paper investigates the problem of an ejector used in a pressure recovery system for chemical oxygen-iodine laser(COIL). For the purpose to reduce the scale of an ejector as well as to increase pressure ratio prec/p, it is better to use the eject gas having small molecular weight and specific heat ratio. Another method to increase the pressure ratio prec/p is to increase the temperature and Mach number of the eject gas.