On the basis of the coexistence theory of slag structure. the thermodynamic calculating model of CaO-B
2O
3 slag melt was deduced. The agreements between calculated
NCaO,
NB2O3 and measured
acao aB2O3 are well. The relationship of nitrogen solution in CaO-B
2O
3 slag with mass action concentrations of structural units has been suggested and the related physical means are discussed. The nitrogen solution in CaO-B
2O
3 slag is closely connected with the slag structure.