Abstract: The behaviors of mineral matters in Shengli and Gaoping coal ash under reducing atmosphere at high temperatures were investigated by X-ray diffraction (XRD). The contents of crystalline minerals and amorphous matters were calculated using Siroquant software based on the XRD pattern of coal ash, and the chemical compositions of amorphous matters in coal ash at high temperatures were studied using the subtraction method. The results show that the content of crystalline minerals and amorphous matters in coal ash and the variation of chemical composition of amorphous matters in coal ash at different temperatures could be obtained using Siroquant and chemical analysis. The form of amorphous matters in coal ash varies with temperature. The amorphous matters in the coal ash exist in the form of non-crystalline oxides below 1100℃ and they transform to vitreous materials above 1200℃ because some crystalline minerals melt at high temperatures. The low Si/Al ratio of coal ash is beneficial to the formation of mullite at high temperature.