Fine porous and hollow ammonium perchlorate (AP) particles were prepared by the spray-drying method. Propellants prepared with porous or hollow AP were found to have bubble contamination. The bubble in the propellant appeared inside the porous and hollow AP particles because the voids in porous and hollow AP cannot be completely filled with HTPB. The relationship between the burning rate and the weight mean diameter, D-w, and the specific surface area, S-w, is divided into two regions. ne burning rate was almost constant above the critical D-w and increased with decreasing D-w below that. The burning rate was almost constant below the critical S-w and increased with increasing S-w above that. These critical points did not depend on the voids in the AP particles. The burning rate of the propellant prepared with spherical AP was dependent on D-w and S-w. The burning rates of the propellants prepared with porous or hollow AP were not associated with D-w or S-w alone and were greater than that of the propellant prepared with spherical AP at a constant D-w or S-w, The voids in porous and hollow AP particles thus had a positive effect on the burning rate.