The combustion of a metallized composite solid propellant containing boron particles is numerically studied. The mathematical model is based on the approaches of two-phase reacting medium mechanics for describing processes in a two-phase flow above the propellant surface and the Hermance model for describing the decomposition of a composite solid propellant. The model of combustion of a metallized composite solid propellant takes into account the oxidation and combustion of boron particles. The dependence of the burning rate of a metallized composite solid propellant on the gas pressure above the combustion surface and the size of boron particles is determined from a parametric study.