FactSage has been used to predict the ash behavior and ash fusion temperature (AFT) at high temperature under reducing atmosphere conditions. For Huainan coal ash samples, it has demonstrated a good agreement between the liquid phase formation as the function of temperature and the tested AFT. The tested and predicted flow temperature (FT) for two typical Huainan coal ashes with the addition of CaO flux are quite fit with the maximum temperature difference less than 74 degrees C which is within acceptable range. It can be concluded that FactSage in combination with X-ray diffraction (XRD) can be used to predict the reactions occurring between minerals, as well as the mineral transformation and slag formation. This is probably an improved way to interpret melting properties of mineral matter in coal and assist in quantifying slag formation in gasifier operation.