Abstract: To meet the requirement of gasification of Shanxi anthracites with slag tapping, the effects of CaO, MgO and Fe₂O₃ flux on the improvement of ash fusibility and viscosity-temperature property were evaluated and compared. The results show that for high silicon and aluminum coal ash with the Si/Al ratio from 1.2 to 2.0, the order of fluxing effect is MgO > CaO > Fe₂O₃. The difference of fluxing effect is caused by the different stable minerals formed at high temperature. For three different fluxes, the relation between the flow temperature (FT) and the liquidus temperature (t_liq) is determined. And a linear relationship between CaO or Fe₂O₃ content and FT is obtained as follows: FT= 1 593-9.573 × w_CaO (R²=0.942 9) and FT = 1 576-8.330 6 × w_Fe2O3 (R²=0.955 9), which are useful to guide the addition of flux. CaO, MgO and Fe₂O₃ show different effects on the viscosity-temperature character. Judging from the viscosity value and the temperature of critical viscosity, CaO displays the best performance as a flux. The different electronegativity of Ca²⁺, Mg²⁺, Fe²⁺ and the formation of different minerals with addition of CaO, MgO and Fe₂O₃ at high temperature are responsible for the various influences of flux on the viscosity value. Small ion radius of Mg²⁺ and Fe²⁺ and the possible crystallization of iron at high temperature are the reasons for the higher temperature of critical viscosity.