Notched specimens of four stress concentration factors K _t =1.2, 2.0, 2.93 and 3.77 were made by machining process using a 500 MPa class steel plate for hull structures. Fatigue tests were performed on the specimens under a constant amplitude load and a stress ratio of 0.1 in air and in synthetic seawater. In particular, corrosion fatigue tests were carried out under a cycling rate of 0.17 Hz, a temperature of 25°C and natural corrosion. The crack initiation life, Nc, was defined with the throughthickness crack depth of 1 mm. The effects of the stress concentration and the maximum stress at the notch tip on the crack initiation life were discussed. The results were as follows. (1) The S-N curves are expressed as an equation LogNc= A+ BΔS. The linear relationships are found between the constants (A and B) and K_t for K_t<3. Then the constants are also expressed as functions of K_t. The S-N curve is, therefore, estimated for an arbitrary Kt less than 3. (2) The fatigue limit decreases linearly as K_t increases for K_t≥3.1 in air. The fatigue limit becomes a constant value for K_t≥3.1, and that is estimated from the equivalent stress of which the maximum stress at a notch tip becomes the upper yield point. (3) On the relationship between the equivalent stress at the notch tip, S_eq, and N_c, the crack initiation behavior is classified into three regions ; (a) the maximum stress at the notch tip is more than the upper yield point, (b) that is between the upper yield point and the lower yield point, and (c) that is less than the lower yield point. (4) For the above region (a), the slope of the S_eq/N_c relation is almost the same in air and in seawater, and then the mechanism of crack initiation behavior is no significance difference in both environments. The decease of crack initiation life in seawater is cased by the difference of crack growth rate. (5) For the above region (b), a specimen has non propagating cracks in air, and the corrosion fatigue failure occurs by the dissolution of a crack tip in seawater. (6) For the above region (c), the fatigue crack initiates after the growth of a corrosive pit.