Fatigue strength data of notched specimens of steels were analyzed by distinguishing the fatigue limit for crack initiation, σω1, and that for fracture, σω2, with the aid of fracture mechanics. The data were extracted from Data Book on Fatigue Strength of Metallic Materials published recently by Japan Society of Materials Science and JSME Data Book, entitled Design Data on Fatigue Strength of Metals (I)published by Japan Society of Mechanical Engineers about 20 years ago.Although there existed little difference in characteristics of fatigue strength data between the above-mentioned two data books, empirical formulation of fatigue strength reduction factor, β, proposed by JSME was found to be unsatisfactory in the sense that σω1 and σω2 were not distinguished and to give an unconservative estimation for the size effect on σω2 as a design criterion.As for the σω1, since machining conditions of test specimens might give a large effect on fatigue strength, it seemed difficult to derive better information from this kind of data books other than the one that the stress concentration factor, α, provides the upper limit of β1.As for the σω2, the data are urgently needed for large sized specimens to elucidate the size effect on σω2 and to discuss the possibility of applying fracture mechanics to fatigue of notched specimens.