In order to investigate the effects of small defects and nonmetallic inclusions on the fatigue strength of high-strength steels under tensile or compressive mean stress, fatigue tests for a maraging steel with the static strength level of 2500MPa (H_v=740), were carried out. The experimental data on a 0.13 % C steel (H_v=105) as well as the maraging steel are analyzed from the viewpoint of the previous study in which the prediction equation for the fatigue strength under zero mean stress (R=-1) was proposed on the basis of the Vickers hardness H_v and the square root of the projection area (√(area)) of a defect or nonmetallic inclusion. The new equation is proposed to predict the fatigue strength under tensile or compressive mean stress. The proposed equation gives not only accurate prediction of the fatigue strength of a high-speed cutting steel SKH-51 (H_v=615, 654) which fractured from nonmetallic inclusions, but also can be used to predict the lower limit of the wide scatter of the fatigue strength of many specimens.