SiO2/Al2O3 (S/A) in the ash content is always regarded as the key factor on ash sintering and slagging in the boiler and gasifier. However, the effect of S/A on ash fusion behavior is not clear for using improper samples and few quantified analysis of fusion process has been conducted. In this work, the effect of S/A has been investigated by ash fusion temperature (AFT) test, thermomechanical analysis (TMA) and thermal dynamic calculation. The composition of samples is designed for illustrating the effect of S/A, and excluding other factors. We investigated the fusion behavior of two groups of synthetic ashes with acid oxide (SiO2 + Al2O3) of 77% and 70% respectively. The ash fusion behavior in three different stages is influenced by S/A for different initial melts, slag formation rate, viscosity of liquid phase and then the solids left. The onset temperature of sintering stage (T-on) decreases with S/A until 2.0 and then remains stable. T-on is closely related to T-10 (temperature when slag above 10%). In sintering stage, the sintering rate is influenced by viscosity of liquid slag for that the liquid phase is limited in this stage. However, in primary fusion stage the shrinking rate is dominated by liquid phase only when the slag viscosity is not high enough, so the shrinkage of ash samples with low S/A is following the trend of slag formation rate by Factsage due to the low viscosity of liquid phase formed. Besides, samples with S/A higher than 3.5 can be considered as Newtonian fluid at FT due to low solid content. (C) 2016 Elsevier Ltd. All rights reserved.