The focus of this work is on the use of thermal analysis to understand the influence of the particle size on the decomposition of KClO4. DSC curves of KClO4, with different particle sizes, show an endothermic peak from a phase transition close to 308 degreesC and a melting point at 608 degreesC. DTA curves gave similar results. Also observed is the fusion of KCl at about 770 degreesC. The corresponding TG curves of decomposition for the reaction of KClO40 --> KCl + 2O(2) were recorded and the percentage of mass loss is close to the theoretical value (46.18%). The initial decomposition temperature of KClO4 with smaller particle sizes is shifted to lower temperatures, and the decomposition activation energies of KClO4 range from 231 to 269 kJ mol(-1).