The burning rate of AP/HTPB composite propellant increases with increasing AP content and with decreasing AP size. In addition, the burning rate can be enhanced with the addition of Fe2O3. The burning characteristics and thermal decomposition behavior of AP/HTPB composite propellant using coarse and fine AP particles with and without Fe2O3 at various AP contents were investigated to obtain an exhaustive set of data. As the AP content decreased, the burning rate decreased and the propellants containing less than a certain AP content self-quenched or did not ignite. The self-quenched combustion began at both lower and higher pressures. The lower limit of AP content to burn the propellant with coarse AP was lower than that with fine AP. The lower limit of AP content to burn was decreased by the addition of Fe2O3. The thermal decomposition behavior of propellants prepared with 20–80 % AP was investigated. The decrease in the peak temperature of the exothermic decomposition suggested an increased burning rate. However, a quantitative relationship between the thermochemical behavior and the burning characteristics, such as the burning rate and the lower limit of AP content to burn, could not be determined.