The sericite ore is formed by the hydrothermal alteration of rhyodacitic welded tuff. The alteration zone of the host rock can be classified into four types based on the mineral assemblages ; sericite, quartz-sericite, silicified and propylite zone. The sericite ore mainly occurs as vein types and fault clay along the fault plane in the quartz-sericite zone. Mineral components of the sericite ore are mainly sericite with minor diaspore, corundum and pyrite. The sericitic porcelaineous ore is mainly composed of quartz and sericite. Accessory minerals are muscovite, diaspore, sphene, corundum, pyrite, iron-oxides and etc. The chemical compositions of K2O, Al2O3, and ignition loss in the sericite ore increase largely than that of the host rock, while the compositions of SiO2, Na2O and Fe2O3 decrease. XRD patterns of the heat-treated sericite ores show the formation of mullite at 1,200℃. and the diaspore-bearing sericite ore forms mullite and corundum at 1,200℃. The differential thermal analysis of the sericite ores show small endothermic peak at 645~668℃. and the diaspore-bearing sericite ore shows a strong endothermic peak at 517℃. It indicates that the decomposition of diaspore appear at lower temperature than that of sericite. The thermal expansivity of the sericite ores show the similar pattern. The sericite ores show the thermal expansivity of 3.3~4.7% at 900℃ and 0.39~0.75% at 1,200℃, respectively. DTA-TG curves of the sericite ores show closely relations with the thermal expansivity.