Abstract: By using the advanced instrumentation of X-ray diffraction (XRD), the mineral matter of three coals with different fusion temperatures and mineral transformation of three coal ashes in reducing conditions were studied. The results show that the coal mineral phases generally include kaolinite, quartz, calcite, gypsum and pyrite. The mineral phases of coal ashes which are ashed at 815 ℃ generally include quartz, hematite, and anhydrite. As the heating temperature increases, the content of quartz, anhydrite and hematite in coal ash will decrease in reducing conditions and some new mineral phases are formed during the heating processes. The new phase of mullite formed is the main reason of high fusion temperature of coal ash samples. Otherwise, the hedenbergite formed at 1 100 ℃ in the heating process is the main reason for lowering the ash melting temperature.