Effects of artificial small defects (drilled hole, shallow notch and the indentation of the Vickers hardness) on the fatigue strength of a maraging steel which received various aging treatments for the attainment of various hardness levels were studied. The fatigue strength σw can be predicted by σw = 0.65(HV+960)/(√(area))1/6………(a) where HV is the Vickers hardness number and √(area) is the square root of the projected area of the artificial defect onto the plane normal to the direction of the maximum tensile stress. Equation (a) explains the reason of relatively low fatigue strength σw0 of maraging steel even with high hardness, because the maraging steel used in this study contains spherical inclusions of TiC with approximately 10 μm diameter and the value of √(area) of TiC was estimated to be about 9 μm by the investigation of the microstructure. Substituting this value and HV into Eq. (a), we can obtain the values of σw0 which are in good agreement with the experimental results. These results suggest that the improvement in fatigue strength of maraging steel and hard steel may be achieved from the viewpoint of Eq. (a).