Abstract: In accordance with the coexistence theory of slag structure and phase diagrams, the structural units of MnO-TiO₂ slag system have been determined as simple ions Mn²⁺,O^2- and molecules TiO₂, 2MnO·TiO₂, MnO· TiO₂ and the thermodynamic calculating model of mass action concentrations has been deduced.The results show the calculated values N_MnOare in agreement with measured α_MnO. Therefore the above model is reasonable. Furthermore the changes of △H^M△G^M with ∑n_MnO are also calculated.