Abstract The effects of wet grinding on kaolin minerals were examined by X-ray diffraction method, differential thermal analysis, electron microscopy and density measurement. The effects of wet grinding of kaolin minerals are different from those of dry grinding, particularly at the early stage of grinding. It has been considered that the force of dry grinding acts as fracturing and cleaving forces, while the force of wet grinding acts as a weak cleaving force for kaolinite and as a weak fracturing force for halloysite in the early stage of grinding. It has been revealed that the effects of wet grinding depend on the shape of crystal in the original kaolin mineral in the early stage of grinding, and beyond a certain stage of grinding, they depend on the structural per-fectness of the kaolin layer, that is, the internal degree of crystallinity of the original kaolin mineral.