This paper presented fatigue strength criterion for ship structural members subjected to still water bending loads and wave bending loads under service conditions. Assuming exponential distribution for long term distribution of wave bending stress during 20 years (10⁸ stress cycle), and using ordinary S-N_c curves ( N_c : crack initiation life) of notched specimens of mild steel and the modified Miner rulue for fatigue crack initiation, the following equations were proposed for the fatigue strength criterion. S_a+0.15S_m=14.4 (kg/mm²) for longitudinal strength members Where, S_a : nominal stress amplitude (kg/mm²) (wave bending stress at a probability level of Q =10^-8) S_m : nominal mean stress (kg/mm²) S_a+0.3S_m=23.6 (kg/mm²) for transverse strength members Where, S_a : true stress amplitude considered stress concentration at a probability of Q =10^-8 S_m : true mean stress considered stress concentration Applying the above formulae to structural members of several ships, reasonable results were obtained.