Abstract: The effects of MgO contents in slag system CaO-SiO2-Al2O3-MgO-CaF2 on the number, size and composition of inclusions in H13 steel deoxidized with Mg-Al-Fe alloy were studied. Results show that when the content of MgO in the slag was 5wt%, the minimum number of inclusions was 55.62 per square millimeter, and the size of all inclusions was less than 8 μm. When the content of MgO in the slag was less than or more than 5wt%, there was more number of inclusions in the steel, and the size of some inclusions was 8-10 μm. When the content of MgO in the slag was 5wt%, 8wt%, 12wt%, respectively, the inclusions were MgO·Al2O3 and Al2O3-SiO2, MgO·Al2O3 and Al2O3-SiO2-CaO, MgO·Al2O3 and Al2O3-SiO2-CaO-MgO, respectively. A stability diagram of inclusions corresponding to magnesium and alumina contents in the steel was calculated employing available thermodynamic data, and the A-type inclusion composition experimentally obtained well agreed with the diagram.