Abstract: To investigate the evolution of inclusions in Si-Mn deoxidized steel during continuous casting, samplings of liquid steel in refining and casting of the commercial steel SWRH82B were conducted in a domestic steel work. Varied features of inclusions in steel, such as type, size and composition etc., were analyzed by an automatic SEM machine. The results showed that re-oxidation of molten steel occurred during RH departure and tundish casting, resulting in pick-ups of total oxygen TO and nitrogen N contents in steel. During secondary oxidation in the tundish, the CaO-SiO2-Al2O3 inclusions can’t form spontaneously and were classified as exogenous inclusions. When re-oxidation occurred in the tundish with TO content increased from 0.0020% to 0.0023%, the changes of Gibbs free energy viz. ΔG of the reaction between Ca and O was lower than that of reactions between Si, Mn and O, resulting in a small amount of CaO constituent in the MnO-SiO2-Al2O3 system inclusions. When TO approached about 0.0027%, the ΔG for reactions of Al, Si, Ca with O were -68.36 kJ·mol-1, -20.59 kJ·mol-1 and -15.30 kJ·mol-1 respectively, which were much lower than those of reactions between Ca, Mg and O (-2.00 kJ·mol-1 and 111.21 kJ·mol-1, respectively). Accordingly, a large number of MnO-Al2O3-SiO2 inclusions were formed at this stage, accounting for a number fraction about 84.92% of inclusions. In the mold, collisions and aggregations between entrapped mold flux particles and existing inclusions caused an increase in the (CaO)/(SiO2) ratio in inclusions, which is close to the binary basicity of mold flux (R=0.75). The proportions of CaO-SiO2-Al2O3 inclusions in steel samples taken from casting mold increased significantly. Thermodynamic calculations indicated that during tundish casting, as the temperature T decreased from 1758 K to 1597 K, the ΔG ranges for the reduction of SiO2 and MnO by Al were -131.25 kJ·mol-1~-115.33 kJ·mol-1 and -35.57 kJ·mol-1~-28.95 kJ·mol-1, respectively. This demonstrated that Al reduced SiO2 and MnO in inclusions, resulting in increased Al2O3 contents in inclusions in the mold. The thermodynamic calculation results were consistent with the analysis results of industrial samples.