Abstract: The morphologies and compositions of typical inclusions in liquid steel before and after calcium treatment were observed to study the evolution mechanism of non-metallic inclusions during calcium treatment. The process of calcium treatment could be modified by thermodynamic analysis. Results show that irregular solid inclusions finally change into spherical inclusions after calcium treatment. When the mass fraction of aluminum in liquid steel is 0.03% at 1873 K, the mass fractions of oxygen and calcium should be controlled in the range of 5×10^-6~17×10^-6 and 0.7×10^-6~30×10^-6, respectively. Inclusions in liquid steel can be modified effectively. When the mass fraction of sulfur in liquid steel at 1873 K is in the range of 6×10^-6~19×10^-6, liquid calcium aluminate inclusions can be formed via Al₂O₃, and the formation of CaS can be decreased.