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+,O2- 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 NMnOare in agreement with measured αMnO. Therefore the above model is reasonable. Furthermore the changes of △HM△GM with ∑nMnO are also calculated.