The viscosities for the selected CaO-MgO-SiO_2 and CaO-MgO-Al_2O_3-SiO_2 slags were measured by rotating cylinder method in a wide temperature range from 1 608 K to 1 823 K.The effects of temperature,slag basicity and Al_2 O_3 content were studied.The results indicated that the viscosity decreased with increasing the slag basicity at the same temperature.And the viscosities of the slag initially increased linearly with increasing Al_2O_3 content in the slags and then decreased as the Al_2O_3 content in the slags continued to increase.Al_2O_3 addition has a more significant effect on the viscosity than slag basicity in the selected slags.Based on the experimental data and the fraction of solid phases calculated by the thermodynamic software,the effect of solid phases on slag viscosity was evaluated.The results indicated that the relationship between the relative viscosity and the volume fraction of solid phases estimated from the slag composition can be reasonable by allowing the inverse maximum fraction of solid particles to vary. The viscosities for the selected CaO-MgO-SiO_2 and CaO-MgO-Al_2O_3-SiO_2 slags were measured by rotating cylinder method in a wide temperature range from 1 608 K to 1 823 K. The effects of temperature, slag basicity and Al_2 O_3 content were studied. The results indicated that the viscosity decreased with increasing the slag basicity at the same temperature. And the viscosities of the slag initially increased linearly with increasing Al 2 O 3 content in the slags and then decreased as the Al 2 O 3 content in the slags continued to increase. Al_2O_3 addition has a more significant effect on the viscosity than slag basicity in the selected slags. Based on the experimental data and the fraction of solid phases calculated by the thermodynamic software, the effect of solid phases on slag viscosity was evaluated. the relationship between the relative viscosity and the volume fraction of solid phases estimated from the slag composition can be reasonable by allowing the invers e maximum fraction of solid particles to vary.