In this study, ammonium perchlorates (APs) with two or more different particle sizes were mixed to optimize the viscosity of the propellant slurry and the burn rate of the resulting solid composite propellants at various mixing ratios. First, to investigate the characteristics of bimodal AP, APs with 200 and 6 mu m particles were used. The experimental results showed that the viscosity of the propellant mixture was excellent when the AP-200 content, i. e., the AP with 200 mu m particles, ranged from 55 to 70 wt%, and the burn rate increased with the increasing AP-6 content. To investigate the characteristics of trimodal AP, three types of APs with 400, 200, and 6 mu m particles were used. The findings revealed that the mixing ratio of the three types of APs most favorable to the rheological properties was approximately 1 : 1 : 1. In addition, although adjusting the ratio between large and small particles was desirable for changing the burn rate, varying the ratio between AP-400 and AP-200 did not significantly change the burn rate. Adjusting the AP-6 ratio effectively changed the burn rate. The method of adjusting the AP contents, i. e., the solid contents of propellant, can effectively alter the viscosity and burn rate of propellant. When the total AP content was reduced, the burn rate decreased, but the overall propellant performance degraded. However, the performance parameters of the propellant, such as its density and specific impulse, could be improved by adding solid fuels, such as aluminum.