Viscosities of selected quaternary CaO-MgO-Al2O3-SiO2 slags have been measured by rotary cylinder method up to a temperature of 1875K. 9 slag compositions have been selected in the composition range 40.04-50.64 wt. % CaO, 6.40-7.12 wt. % MgO, 22.84-41.73 wt. % Al2O3 and 10.25-20.70 wt. % SiO2. The effect of increasing temperature was investigated and it was shown that viscosity decreased, as expected, with increasing temperature. The viscosity increases with increasing the ratios of CaO/Al2O3, and following by a decrease with further increasing the ratios of CaO/Al2O3. An application of Iida’s viscosity model has been made. The Iida model gave good consistency with the measured viscosity values.