Abstract The effects of several-hundred-hour mechanical-mortar dry grinding of kaolinite were studied by X-ray, thermal, electron microscopic and other methods. It has been found that there are two sorts of structural change in the process of dry grinding of kaolinite. One is the production of a non-crystalline substance attended by the disordering of the crystalline part, and the other is the reaggregation process. The process of the reduction in the particle size and the process of the production of the non-crystalline substance are connected to the process of the reaggregation. In a certain stage of the dry grinding, the reaggregates are spherical particles which have a zeolitic structure. As the grinding further progresses, the structure of the crystalline part in this radial particle becomes disordered due to the mechanical stress, and it changes into an amorphous substance at last. Consequently, the effect of dry grinding of kaolin mineral depends on the structural perfectness of alumino-silicate layers of the kaolin mineral, that is, the internal crystallinity of the kaolin mineral.