In order to evaluate precisely the life and residual-life time of ship structures, it is essential to examine the S/N relationships of their fatigue strengths. The long term corrosion fatigue behaviors, however, have not been fully investigated yet. In the present study, we have measured the corrosion rates and the fatigue crack growth rates of a 500 MPa class steel plate in synthetic seawater, and discussed the long term corrosion fatigue strength obtained under a cyclic frequency of 0.17 Hz within stress ranges less than the fatigue limit tested in air. The results obtained are as follows. (1) The corrosion rate, R_c , in an immersion test was 0.07 and 0.09mm/year for a polished and shotblasted surfaces of the plate, respectively. The effect of the stress increase in corrosion wastage cannot be neglect in the corrosion fatigue tests exceeding five years under the cyclic frequency of 0.17 Hz. (2) The crack growth rate measured in seawater was two to three times larger than that in air in the range where Paris equation may hold. The effect of the environment on a crack growth rate is significant in the range where the crack intensity factor is relatively low. (3) The range of the threshold stress intensity factor, Δ K_th , is 2.5 and 1.2 MPa m^1/2 in air and in seawater, respectively. (4) The long term corrosion fatigue strength can be calculated roughly from the empirical equation.Δ K = f ( R_c , y ) C_cNⁿ_c , where f ( R_c , y ) is the corrosion wastage rate estimated from the corrosion rate of material and year, N_c is crack initiation life, n is the index given by crack growth rate, and C_c is a constant obtained from the corrosion fatigue tests of about 10⁶ cycles. (5) For the long term corrosion fatigue, according to the results of Komai and coauthor, no fatigue cracks initiate from the corrosion pits after 14 to 24 years in the plate specimen area of 320 mm² with R_c =0.09 mm/year, and the pits move to the progression of corrosion wastage in corrosion fatigues less than Δ K_CF . (6) In order to decide the f ( R_c , y ) during corrosion fatigue, the tests through five years are necessary.