The heterogeneous propellants consisting of Ammonium Perchlorate (AP) particles imbedded in a Hydroxyl Terminated Polybutadience (HTPB) are widely used in solid rocket industries. In this study, the effect of AP particles diameter and the random distribution with the HTPB on the burning of such propellant is presented. A mathematical model which describes the unsteady burning of a heterogeneous propellant by simultaneously solving the combustion fields in the gas phase and the thermal field in the solid phase with appropriate jump condition across the gas/solid interface is developed. The gas-phase kinetics is represented by a two-step reaction mechanism for the primary premixed flame and the primary diffusion flame between the decomposition products of the HTPB and the oxidizer AP. The propagation of the unsteady non-planer regression surface is described, using the Essentially-Non-Oscillatory (ENO) scheme with the aid of the level set strategy. The results show that the large AP particle diameter has a marked effect on the combustion surface deformation and on the burning rate as well. Therefore, modeling of the condensed phase process is mostly conducted on AP. Moreover, the effect of various parameters on the surface propagation speed, flame structure, and the burning surface geometry is obtained.