The viscosities of CaO-SiO 2 -10wt%MgO-20wt%Al 2 O 3 -Na 2 O slags (CaO/SiO 2 ¼ 0.8-1.2, wt%Na 2 O ¼ 0-10) were measured to estimate the effect of Na 2 O on the viscous behavior at elevated temperatures. The slag viscosity at 1773 K decreased with increasing Na 2 O contents, which is observed in a basic oxide component. However, the FT-IR spectroscopic analysis of the slag structure seemed to verify that [AlO 4 ]-tetrahedra depolymerized more readily than [SiO 4 ]-tetrahedra due to ionic charge compensation which has a tendency to be more effective on [AlO 4 ]-tetrahedra. Hence, for the present slag system, the viscosity decreased with additions of Na 2 O by providing network breaking O 2ions, and the charge balance by Na þ ions independently contributed to the depolymerization of the network structure of the molten slag. In addition, Na 2 O also lowered the critical temperature (T CR ) resulting in a larger temperature difference between the operating temperature (T OP ) and T CR , which can enhance operational stability.