Numerical solutions of full Navier-Stokes equations were obtained for a nozzle two-phase flow of the nitrous oxide(N2O)/hydroxyl-terminated polybutadiene(HTPB) based fuels hybrid rocket motor.MacCormack decomposition scheme and B-Lalgebraic turbulence model and two-phase equilibrium flow model were used.The nozzle flow and performance under three oxidizer-to-fuel ratios and four nozzle types were calculated.Effect of oxidizer-to-fuel ratio and nozzle type on nozzle performance was analyzed from the two-phase and gas phase results.Vacuum specific impulse is bound up with oxidizer-to-fuel ratio.This paper will be useful for the hybrid rocket motor design.