The steady-state combustion of the monopropellant glycidyl azide polymer (GAP) has been modeled using a one-dimensional, three-phase numerical model. Combustion characteristics of four formulations of cured GAP with varying amounts of the curing agent hexamethylene diisocyanate (HMDI) have been modeled. A two-step global decomposition condensed-phase kinetic mechanism has been developed, based on experimental data. A detailed gas-phase kinetic mechanism, with 460 reactions and 74 species, has been assembled and used. The combustion has been modeled over pressures of 5-100 atm and initial temperatures of 298 50 K. The calculated combustion characteristics include the burning rate, pressure exponent, temperature sensitivity, surface and flame temperatures, temperature and species profiles, and condensed-and gas-phase heat released. The model calculations have been compared with various experimental data, and most of the calculations and their trends seem to be consistent with experimental data. The GAP content of the GAP-HMDI formulation has been predicted to have a significant influence on the burning rate. The calculated GAP-HMDI burning rates were similar to 1.05-1.93 cm/s at 70 atm, increasing with the GAP content of the formulation. The calculated pressure exponents were similar to 0.4 and the calculated temperature sensitivities were similar to 0.01-0.014 K-1. The condensed phase plays a significant role for GAP combustion. Parametric studies have been performed to study the effect of varying the values of reaction parameters and thermophysical properties such as specific heat and thermal conductivity.