The authors have newly designed and manufactured a large rotating bending fatigue test machine (Cantilever type, Max. bending moment 7 t-m, Standard diameter of specimen 125 mmφ) and tested specimens of 135 mm φ under sea water corrosion. The specimens have been taken from steel bars forged from 1. 4 ton ingots made by acid open-hearth process (forged ratio = 6) and annealed in the furnace. The chemical and mechanical properties of the specimensare as shown below : Chemical Properties Mechanical Properties C 0.22 Tensile strength (kg/mm²) 46.447.0 Si 0.23 Yield point (kg/mm²) 24.024.5m Mn 0.42 Elongation (50 mm, %) 36.837.2 P 0.013 Reduction of aea (%) 58.163.2 S 0.043 10 mm φ specimens of the same material having simillar shape have been also tested under sea water corrosion for comparison. The following facts have been revealed by the tests : (1) Fatigue strengths of 130 mm φ and 10 mm φ specimens at 10⁸ stress reversal were 11.2kg/mm² and 3. kg/mm² respectively. (2) Depth of cracks found on the corroded surface of specimens is calculated by the following formula : l =C×log₁₀(N/Nc) where l : Depth of corrosion cracks (mm) C : Constant, determined by the method of corrosion (=0.86 in this experiment) Nc : Limit of stress reversal at which corrosion cracks will originate (=1.7×10⁶ in this experiment) N : No. of stress reversals. (3) When N does not exceed 1.7×10⁶, cracks will not originate. (4) A reversal stress limit (σ _c ) at which surface cracks have originated on large corroded specimens may be pressumed about 5kg/mm². (5) There is also a reversal stress limit (σ₀) at which fine cracks on the non-corroded specimens begin to develope, σ₀ is assumed about 11 kg/mm² for the steel tested (6) Failure of corroded specimens may occur when reversal stress has increased as the result of reduction of effective area by surface cracks and exceeds σ₀. (7) Fatigue limit of large corroded specimens is designated by σ₀. (8) There may be no fatigue limit for small corroded specimens.