Limestone is widely used in entrained flow gasifiers to improve slag viscosity properties, while viscosity-temperature curve of slag is typical of a crystalline slag when CaO content is above a certain level, of which the viscosity sharply increases when the temperature is lowered below temperature of critical viscosity (TCV). Experiments were conducted on selected slags at about 5 wt% Fe2O3 level to investigate effect of CaO on the pattern of viscosity-temperature curve (PVTC) of slag. The results showed that the higher CaO content at which the viscosity of slag exhibited the behavior of a crystalline slag increased as silica/alumina ratio (S/A) of slag increased. PVTC of slag was essentially depended on the increase of solid amount with decreasing temperature. Although solid amount at high temperatures was lowered due to addition of CaCO3, its average increase rate (Rs) increased with increasing CaO content. When anorthite and melilite were the dominant solid formed in slag, the viscosity of slag will exhibite the behavior of a crystalline slag once Rs exceeded 0.250 wt%/degrees C. However, effect of CaO content on Rs diminished with increasing S/A when Rs was less than 0.250 wt%/degrees C. Rs is able to be used as the indication for using limestone as flux for slag tapping. (C) 2016 Elsevier Ltd. All rights reserved.