HMX is an important energetic ingredient in solid propellants and explosives. For safe and effective use of HMX in various applications, it is essential that the burning characteristics are properly understood. In this study, the self-deflagration of HMX monopropellant is investigated using a one-dimensional steady-state multiphase combustion model. Decomposition in liquid phase is modeled using a detailed reaction mechanism, consisting of elementary reactions investigated using quantum mechanics calculations, as opposed to a global three-step mechanism used till date in previous combustion models. Phase transformation in the solid region from β-HMX to δ-HMX is also modeled and the thickness of the δ-HMX region is calculated in addition to the thickness of the melt layer. Model inputs in the liquid phase are rigorously calculated using quantum mechanics calculations and molecular dynamics simulations. The simple model, with homogeneous liquid phase and dissolved gases in the melt layer, is able to predict the burn-rate and its temperature sensitivity as well as the melt-layer thickness. Predicted temperature and species profiles are also in reasonable agreement with the experimental measurements. Various combustion wave characteristics investigated at 1 atm in the present study are also being investigated over a wide range of pressures.