The effects of Mg on the evolution of non-metallic inclusions in Mn–Si–Ti deoxidised steels during solidification were investigated in a study based on experiments and thermodynamic calculations. The inclusions were composed of the MgO–MnO–Ti2O3–TiO2 oxide, MnS, and TiN. With the increase of Mg concentration in steels, the phases of oxide inclusions were changed, in the order of pseudobrookite (Ti3O5–MnTi2O5), ilmenite (MgTiO3–MnTiO3–Ti2O3), spinel (Mg2TiO4–MgTi2O4–Mn2TiO4–MnTi2O4) and MgO. Thermodynamic calculations for inclusion evolutions were in good agreement with the experimental results.