We investigated the effect of Al2O3 content on the viscosity of CaO–SiO2–Al2O3–8wt％MgO–1wt％Cr2O3 (mass ratio of CaO/SiO2 is 1.0, and Al2O3 content is 17wt％–29wt％) slags. The results show that the viscosity of the slag increases gradually with increases in the Al2O3 content in the range of 17wt％ to 29wt％ due to the role of Al2O3 as a network former in the polymerization of the aluminosilicate structure of the slag. With increases in the Al2O3 content from 17wt％ to 29wt％, the apparent activation energy of the slags also increases from 180.85 to 210.23 kJ/mol, which is consistent with the variation in the critical temperature. The Fourier-transform infrared spectra indicate that the degree of polymerization of this slag is increased by the addition of Al2O3. The application of Iida's model for predicting the slag viscosity in the pres-ence of Cr2O3 indicates that the calculated viscosity values fit well with the measured values when both the temperature and Al2O3 content are at relatively low levels, i.e., the temperature range of 1673 to 1803 K and the Al2O3 content range of 17wt％–29wt％ in CaO–SiO2–Al2O3–8wt％MgO–1wt％Cr2O3 slag.