Abstract: A mathematical model describing combustion in a high pressure liquid hyper-golic bipropellant(UDMH/H2O4) rocket engine combustor has been developed. In this model, the droplet evaporation process at high ambient pressure has been considered,which takes into account the droplet surface regression, the transient heating, the effects of ambient pressure on vapor-liquid equilibrium, the effects of nonideal gas behaviour, the dissociation of N2O4 and the decomposition of UDMH. The gas temperature variation has also been considered which is dependent on the local mixture ratio, the drop-size distributions and the effects of the convergent section of nozzle on the vaporization rates.Since the mechanism of the droplet secondary breakup process is different under a large flow flux,a new secondary breakup critical condition is proposed.The application of this mathematical model to different operating conditions of a specific rocket combustor is provided. The calculated results are in good agreement with experimental data. This model correlates the combustion process with the engine structure parameters, the operating parameters and the properties of the propellant. Therefore, it may be used as a guideline and a theoretical prediction for designing rocket engine combustor.