Abstract: A comprehensive theoretical model for unsteady evaporation/decomposition combustion of a monopropellant droplet in inert medium at high temperature and high pressure is developed. It takes into account the decomposition combustion mechanism of monopropellant, the principal effects due to high pressure, nonuniformity and unsteadiness of temperature distribution within the droplet, the liquid continuum equation, the strict requirements for mass, species, energy conservation conditions at liquid-gas boundary and the dependences of properties on physical parameters.The conservation equations of mass, species and energy are solved simultaneously by means of the complete implicit finite difference scheme. The effects of pressure on vaporization behaviours of an UDMH droplet in nitrogen gas at high temperature are calculated. The results show that the effects of pressure are very strong and the pressure limit for the supercritical evaporation/decomposition combustion of the UDMH droplet is about 5 MPa.