Activity of Na₂O in the Na₂O-B₂O₃ slag at 1373K was measured by using thermochemical equilibration technique. The activity of Na₂O increases from 1.86× 10^(-6) to 4.99×10^(-4) with increasing Na₂O content from 11.0 to 64.9% in molar basis. The activities of Na₂O and B₂O₃ in the molten slag indicated that each component shows significant negative deviation from an ideality. None of the peak is observed in the relationship between the stability function and slag composition. The (excess) stability significantly changes at the composition of X_(Na2O) = 0.25 and 0.60, indicating the difference of the network structure of the borate melt at these compositions. The change of the partial molar Gibbs free energy of mixing, theoretical optical basicity, and the lattice energy was introduced to understand the thermodynamic behavior of Na₂O-B₂O₃ melt. As a result, the thermodynamic behavior of the Na₂O-B₂O₃ system with various slags can be explained more quantitatively by the lattice energy in the present study. In addition, the dependence of the activity coefficient of sodium in molten silver on Na content at 1373K can be expressed by the following equation. In γ_(Na) = 11.5(1-X_(Na))²-8.13 (0 $lt; X^(Ag)_(Na) $lt; 0.005).