Combustion simulations at the micro and meso-scales are coupled to give a more theoretically based and accurate description of AP (ammonium perchlorate)/HTPB (hydroxyl-terminated polybutadiene) composite propellant combustion. One and two-dimensional micro-scale combustion models of AP and AP/HTPB, which include detailed kinetics and species transport, are utilized to define semi-global kinetics for a meso-scale propellant combustion model capable of representing the complex morphology of an actual propellant. The AP/HTPB flame structure predicted with the micro-scale models is separated into four flames. Each flame is represented with a single reaction, thus a new four-step kinetic mechanism is implemented into the meso-scale model. Physical and kinetic parameters are determined with the micro-scale models. The meso-scale model is calibrated and validated with the micro-scale models to ensure the correct flame structure, as a function of pressure and AP particle sizes, is recreated with the four-step mechanism. The primary focus of this work is to outline the methodology used to couple the two numerical scales. Results of the meso-scale model are compared with the previous empirically-parameterized meso-scale model results and experimental data. Predictions are within 10% of experimental values for a range of AP/HTPB propellants. Published by Elsevier Inc. on behalf of The Combustion Institute.