Abstract: In order to reveal the mechanism of Na₂O influence on ash fusion temperatures (AFTs), effect of Na₂O on mineral transformation of two coal ashes with different SiO₂+Al₂O₃ levels were investigated by XRD and FT-IR under reducing atmosphere at high temperature. Thermodynamic software package FactSage was used to calculate the ΔG of reactions between minerals to reveal the mechanism of Na₂O influence on mineral transformation. It is found that the effect of Na₂O on mineral compositions depends on SiO₂+Al₂O₃ levels of coal ash. For ash with 82.89% SiO₂+Al₂O₃ while Na₂O content is 5%-20%, albite and nepheline are formed, leading to a decrease of AFTs. However, only nepheline is formed when Na₂O content is higher than 20%. For ash with 47.85% SiO₂+Al₂O₃, when Na₂O content is less than 10%, no Na-containing mineral is observed. When Na₂O content is higher than 10%, Na-containing minerals such as combeite, lazurite and sodium aluminium oxide are formed, resulting in a decrease of AFTs. Furthermore, FactSage results reveal that Na-containing mineral is easily formed at high temperature due to low ΔG of the reactions.