Abstract: From the viewpoints of coexistence theory of the slag structure, the investi-gation on the structural units of MnO-SiO₂ system and the chemical reactions between them has been carried out. The relationships between the physical properties of the slag and the mass action concentrations of its have been set up at certain temperature and slag composition range, which based on the thermodynamic data of the chemical reactions between the units and the method of linear regression. The calculated data from the model are in good agreement with the measured results. These calculated values are more accu-rate and systematic than those from other expirical and semi-expirical formulas used at present.