Abstract: Mechanism of explosive sintering of two kings of high speed steel powder with remarkable difference in frain size is studied, In the course of sintering "grain effect" is found apparently. That is, for the HPS powder with the same composition,small grain is prior to melting than the large. The melting process takes place in peak pin.Melting bed will gradually build along the grain surface with the increase of shoce energy,there by resulting in intensifying fluid motion.accelerating grain melting.Finally grains bind each other,sinter-ing occurs.