Abstract: The theoretical specific impulse formula for two-phase flow in nozzle of rocket-ramjet engine is derived on the basis of similarity to that used for rocket engine.Theoretically,it is proved that for both these engines,the properties of two-phase flow in nozzle can be analysed with a common combined parameter which is equivalent to exhaust velocity.A simple estimation method for specific impulse losses of two-phase flow with variable lag is provided.An approximate solution dealing with the singularity is proposed.