Abstract: The effect of magnesium addition on the number, morphology, composition, size, and density of inclusions in H13 die steel was studied. The results show that the total oxygen content in the steel can be significantly decreased to 0.0008wt%. Al₂O₃ and MnS inclusions are changed into nearly spherical MgO·Al₂O₃ spinel and spherical MgO·MgS inclusions, respectively. The number of inclusions larger than 1 μm decreases and the number of inclusions smaller than 1 μm increases with increasing magnesium content. V(N,C) precipitates around MgO·Al₂O₃ and MgO·MgS inclusions during solidification of liquid steel. The densities of MgO·Al₂O₃ spinel inclusions are lower than that of alumina inclusions. With increasing magnesium content in the Mg-containing inclusions, the density of inclusions decreases, leading to the improvement of inclusion removal efficiency.