Abstract: The fundamental or common thermodynamical relations between calcium-treated aluminium-killed molten steel and non-metallic inclusions were discussed. The thermodynamic properties of low carbon Al-killed steel and inclusions in the CSP process were calculated using the interaction coefficients of activity and the Bjorkvall method respectively. A detailed analysis of the CaO·Al₂O₃ binary system established the modification of solid alumina inclusions via the following intermediate phases:3CaO· Al₂O₃, CaO· Al₂O₃, liquid phase 12CaO·7Al₂O₃ and CaS. The conditions of forming liquid calcium-aluminate inclusions and avoiding the formation of CaS in calciumtreated aluminium-killed molten steel were also discussed using these thermodynamic relations. The results are in good agreement with the plant data.