Temperature at the center of burning char particles was measured. Effects of the temperature and coal properties on behavior of minerals in the coal ash with vitrification and sintering of the ash were discussed. The maximum temperature inside the burning char particle was 20-400 K higher than the surrounding temperature. For the coals with relatively low melting points of ash, the maximum temperature almost linearly decreased with the increased surrounding temperature, while for the coals with relatively high melting points of ash, the temperature increment was almost kept at 200-300 K. For all coals, the temperature increment reached to zero above 1623 K. Quartz peak intensity measured by X-ray diffraction decreased with the maximum temperature and the peak disappeared by vitrification of ash, at about 1673 K irrespective of its melting properties. Effects of the vitrification on leachability of various elements in ash were also discussed.