The dependence of ΔKth on crack size and material properties under stress ratio R=-1 was studied on various materials and microstructures. The values of ΔKth for all the materials investigated were standardized with one geometrical and one material parameter.The geometrical parameter, √area, is the square root of the area which is occupied by projecting defects or cracks onto the plane normal to the maximum tensile stress. The relationship between ΔKth and √area is expressed as follows:ΔKth∝(√area)1/3 (a)The most relevant material parameter to standardize the data was the Vickers hardness, and the following relationship was obtained:ΔKth∝(HV+C) (b)The constant C in Eq. (b) reflects the difference of nonpropagation behavior of small cracks in soft and hard metals.By combining Eqs. (a) and (b), the following equations were derived for predicting ΔKth and the fatigue limit σω of cracked members.ΔKth=3.3×10-3(HV+120)(√area)1/3 (c)σω=1.43(HV+120)/(√area)1/6 (d)where the units are ΔKth: MPa·m1/2, σω: MPa, √area: μm and HV: (kgf/mm2). Equations (c) and (d) are applicable to a crack having √area approximately less than 1000μm.