Abstract: The effects of Mg on the mechanical properties of 35CrNi3MoV steel has been studied in this paper. The results show that Mg has less effect on the tensile properties and hardness of the steel at room temperature, but a right amount of Mg improves impact and fracture toughness and decreases the fracture-transition temperature considerably due to inclusions modification, as well as prolongs the notch stress rupture life dramatically through strengthening grain boundary and modifying boundary carbides. There is an optimum Mg content, which is about 0.0030% in the experimental 35CrNi3MoV steel