Ash fusion temperatures (AFTs) of ashes with different ratio of V2O5 and NiO were investigated under mild reducing (CO/CO2 = 6:4) and oxidizing (air) atmosphere. Fact Sage, X-ray diffraction (X.RD), and scanning electron microscopy/electron dispersive X-ray (SEM-EDX) were applied to determine liquidus temperatures, minerals transformation, and morphology of ash and slag at high temperature. AFTs vary with the content of V2O5, NiO, and VNiO (V2O5 and NiO) and a significant difference was exhibited between reducing and oxidizing atmosphere. Redox reactions of V2O5 and NiO with atmospheres are the major reason for the difference. V2O3 and spinel decrease the melting rate of minerals and increase the AFTs. V2O5 of low liquidus temperature may form eutectic matter with anorthite; so, AFTs significantly decreased under oxidizing atmosphere. Ni aggregation formed regular ball increases AFTs slightly under reducing atmosphere. AFTs with VNiO approach to AFTs with V2O5 because V2O3 is the most refractory minerals under reducing atmosphere and V2O5 forms eutectic matter with spinel under oxidizing atmosphere. The association of refractory minerals and liquid phase influenced the distribution of solid minerals during melting in ash and slag, which was also an important factor to illustrate the influence on AFTs. The correlation between liquidus temperature and AFTs of oxidizing atmosphere for ash containing Ni was established, but the similar correlation for ash containing V was proved not to be feasible. The linear regression relationships of AFTs with the content of V2O5, NiO, and VNiO were established for predicting the fusion temperatures of ash with V and VNi.