Viscosities of synthetic CaO-SiO_2-Al_2O_3 slags were measured by rotating inner-cylinder viscosimeter, and iso-viscosity lines at 1, 500 and 1, 600℃ are drawn on the diagram. Viscosity of CaO-SiO_2 slags at 1, 600℃ decreased with the increase of CaO content and anomaly was not found at the composition corresponding to CaO・SiO_2. By the addition of a small amount of Al_2O_3 to CaO-SiO_2 slag, viscosity was almost unchanged as in the case of the previous investigator, but at more than 20% Al_2O_3 the increase of viscosity became remarkable. In CaO-Al_2O_3 slags, a minimum of viscosity was found on the isothermal line at low temperatures at about 50% Al_2O_3. Viscosities of CaO-Al_2O_3 side slags which had hitherto never been determined were low and comparable to those of the low viscosity region in CaO-SiO_2 side slags. And some considerations on structures of these molten slags were given from the experimental results
