The viscosity-temperature behaviors of ashes with different ratios of V2O5 and NiO were studied. XRD, FTIR, XPS, Raman and SEM-EDX were applied for investigating the mechanism of influences on viscosity by V and Ni based on the formation of crystalline phase and the change of molecule network structure of liquid phase. Under reducing atmosphere, V2O5 was reduced to V2O3 which forms V-O-Al bond with Al2O3. The penetration of Al3+ into the original tetrahedral [SiO4](4-) skeleton was blocked and the framework size of aluminosilicate reduces, so the viscosity of liquid slag decreased significantly. However, the formation of vanadium-rich spinel increased the viscosity of slag and type of slag transformed into crystalline type for spinel precipitating at high temperature. When the temperature was below the melting temperature of Ni, the existence of metal Ni changed slag into crystal type, and viscosity increased sharply. During coexistence of V and Ni, Ni particles were covered by vanadium-rich spinel, which caused T-cv (temperature of critical viscosity) of slag containing VNiO to be lower than that of slag containing V or Ni. (C) 2014 Elsevier B.V. All rights reserved.