This paper presented the fatigue strength criterion for longitudinal strength members subjected to both still water bending loads and fluctuating wave bending loads under service conditions. Using the modified Miner rule, exponential distribution for long term distribution of wave bending stress during 20 years (10⁸ stress cycles) and ordinary S - N _c ( N _c : crack initiation life) curves for notched specimens of mild steel, the following allowable tensile stress S _AL for fatigue strength criterion was given. S _AL = S _a +0.29/ K _t · S _m = f _L / K _t (8.1 f +16.3) kg/mm² where, S _a : stress amplitude (wave bending stress at a probability level of Q =10^-4) S _m : mean stress (still water bending stress) K _t : stress concentration factor ( K _t -values are not to be regarded as absolute values, but as relative values calibrated from experience for structural members) f : usage factor defined as the cumulative fatigue damage ratio summed over a period of 20 years. f _L : σ _B /σ _BM (σ _BM : tensile strength of mild steel. σ _B : tensile strength of another material) Applying the above formula to longitudinal strength members of several ships reasonable results were obtained.