In order to clarify the influence of MgO on the viscosity of blast furnace slag, viscosities of synthetic CaO-SiO_2-Al_2O_3-MgO slags were measured by the use of rotating inner-cylinder viscosimeter as in the case of the first report. Iso-viscosity lines at 1, 500 and 1, 600β are given in the diagrams. When MgO was added to CaO-SiO_2 slag, the viscosity decreased with increasing amounts of MgO up to about 20%, beyond which, however, it increased. The region of low viscosity in CaO-SiO_2-MgO slag was greater than that in CaO-SiO_2-Al_2O_3 slag. When MgO was added to CaO-SiO_2-Al_2O_3 slag, the viscosity decreased, moreover, change in viscosity at constant temperature caused by the change of slag composition became small, but these beneficial effects of MgO were less noticeable than those reported by McCaffery and coworkers. With respect to the fluxing quality, MgO has nearly the same or a little less influence in reducing the viscosity of slags comparing with that of CaO
