Dry rice hull contains 13-29wt% inorganic matter depending on the species, climate and geographic location. The inorganic matter is composed of silica (87-97wt%) with small amounts of alkali and other trace elements. Properties of ash obtained by combustion of between 400° and 1500°C were investigated. The SiO₂ in the rice hull was localized in the outer and inner epidermis of rice hull. The SiO₂ in rice hull ash formed by combustion below 800°C of rice hull powder with a particle size below 152μm was amorphous. Particles of the ash having an average diameter of 20μm were aggregates of small particles with a diameter of 2-5μm. At combustion temperatures above 900°C, the SiO₂ in rice hull ash consisted of cristobalite and a small amount of tridymite. The surface of ash particles melted and the particles bonded to each other. The particle size was 40-60μm. It was found that potassium contained in the rice hull ash caused surface melting of SiO₂ particles and accelerated the crystallization of amorphous SiO₂ to cristobalite.