Drop-weight impact experiments are performed on different particle size Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) granular explosives. The drop-weight impact machine is equipped with the high-speed photographic systems for obtaining a mechanical-chemical image of the impact process. The experimental image shows that the small particles of 700 mu m have melted for a long time and over a large area during the loading process. HMX particle size affects the ignition mechanism. For a single particle, the ignition mechanism changes from friction to viscous heating with the decrease of particle size. The ignition mechanism of 6000 mu m and 1800 mu m HMX particles is friction work. The ignition mechanism of HMX small particles of 700 mu m includes friction work and viscous heat under drop weight impact.