Effects of dry grinding of a mixture composed of kaolinite and aluminum hydroxide in a tumbling ball mill on formation of mullite in a sintered body and its mechanical and thermal properties were investigated by Xray diffraction method, TG-DTA, and measurements of bending strength and thermal expansion coefficient. Grinding of the mixture enables us to induce the structural change of the raw materials from a crystalline state into an amorphous state. About 192 h in grinding time is required to achieve an amorphous mixture, resulting in the formation of a mullite single phase at relatively low sintering temperature. When the crystalline state remains in the ground product, high temperature is needed to form a mullite single phase. The bending strength of the sintered bodies increases with an increase in not only sintering temperature but also grinding time of the mixture. Further, the thermal expansion coefficient of sintered bodies composed of mullite single phase, which is independent of grinding time, is much lower than that of different phases.