To investigate the fracture characteristics of ammonium perchlorate (AP) grains with different sizes within explosive substrate, three kinds of AP/HTPB samples were prepared using three kinds of AP grains with intermediate diameter of 6-8 μm, 30 μm and 300 μm. The samples were damaged by drop hammer impact loading. The samples after impact test were reclaimed and the fracture feature of AP grains was studied using scanning electron microscope (SEM).The fracture characteristics of AP grains with different sizes within explosive substrate under drop hammer impact loading were analyzed. Results show that after impact loading, lots of brittle ruptures of AP grains in three kinds of samples occurr, the shear bands can be seen in some samples, and the bigger the particles, the worse the fracture. The sizes of AP grains after fracture are between 10-100 μm, and the minimum size is less than 10 μm. Combined the shear band theory, the AP rupture characteristic and the rupture size of AP, ones can deduce that under drop hammer impact loading, the brittle ruptures of AP grains take place because of the shear effect of the sample inside, and the fracture size feature of the AP grains is similar to that of the shear bands in shear phenomena of materials.