The Na 2 O-B 2 O 3 system is thermodynamically optimized by means of the CALPHAD method. A two-sublattice ionic solution model, (Na +1 ) P (O −2 ,BO 3 −3 ,B 4 O 7 −2 ,B 3 O 4.5 ) Q , has been used to describe the liquid phase. All the solid phases were treated as stoichiometric compounds. A set of thermodynamic parameters, which can reproduce most experimental data of both phase diagram and thermodynamic properties, was obtained. Comparisons between the calculated results and experimental data are presented.