Abstract: Effect of magnesium addition on the compositions of inclusion was studied in the different oxygen and sulfur contents at 1873 K. Under the present condition,magnesium deoxidation products of low-S content experiment steel can be changed in the order Al₂O₃→MgAl₂O₄→Mg-Al-O-S,and it is consistent with the results of thermodynamics calculation. But magnesium is preferred to react with oxide,comparing with sulfide. It reaches the reaction endpoint after 1 or 5 min,and it is faster than reacting with sulfide. When the content of all elements in high-S content experiment steel satisfy certain conditions,the oxide and sulphide can be modified simultaneously by magnesium. And the complex oxysulfides would be generated. The inclusions in experimental steel can be changed in the order Al₂O₃→MgO→MgO + MgAl₂O₄+ MgS,but the product layer of MgO-MgAl₂O₄depressed the further reaction. Sulphur in molten metal can be fixed in this way,and diameter of the inclusions is well under control at the same time.