Abstract: The effects of Mg on the inclusion characteristics of 35CrNi3MoV steel have been studied. The results show that Mg treatment reduces obviously the amount of inclusion in the steel.Magnesiurn and oxygen in liquid steel form dispersed fine particles of MgO, to which other oxides or sulphides nucleated at heterogeneity and grown during solidification. In this way the inclusions are modified. The small spherical inclusions containing MgO have been observed in the Mg treated steel in contrast to the large angular inclusions with Al2O3 in the conventional steel. Especially the elongated MnS inclusion is replaced by small MgS·MgO or MgS. MnS·MgO complex inclusion. The formation of inclusions have been discussed.