Abstract: The effect of dry grinding of kaolin minerals by a mechanical mortar was examined by x-ray, thermal, and electron microscopic methods. Base exchange capacity and apparent density were also measured. In the early stage of grinding, kaolin crystals cleave and fracture and then split into fine crystals which are considered unit crystallites. Such crystallites gradually change to a disordered kaolin and partially decompose into a noncrystalline substance as grinding progresses. The crystalline and the noncrystalline substances promptly reaggregate and these reaggregated particles have a radial shape. After further grinding, the structure of the reaggregated particle seems to be zeolitic. Finally, the kaolin mineral thoroughly changes to a completely amorphous substance similar to a silica-alumina mixed gel. Consequently, the effect of dry grinding of kaolin is related to the degree of crystallinity of the original kaolin mineral.