Propeller shafts being subjected to repeated stresses are often damaged by the corrosion fatigue when the shaft surfaces are wet by sea water. Some papers concerning the corrosion fatigue of small size test piece have been reported, but it is difficult to clear up the fatigue phenomenon of -actual shaft only from the test result of small piece. To investigate the damage of propeller shaft, large pieces-must be tested in view of the size effect to the corrosion fatigue. The Authors have carried out the torsional corrosion-fatigue test with large pieces of 70 mm in diameter, which are made of 0.22% carbon forged steel. We obtained the following results : (1) The S-N diagram of the corrosion fatigue test indicates that the effect of corrosion appears -after repeated cycles exeed about 106. The corrosion-fatigue limit is 8. 5 kg/mm² (10⁸ repeats), while the fatigue limit in air or oil is 13. 6 kg/mm². (2) The corrosion-fatigue strength of large piece does not indicate so much decrease as the strength of small piecee. (3) The initial twisting stress of 3 kg/mm² does not show any appreciable effect to the test result, when the repeating stress is 11 kg/mm². (4) In case of comparatively high stress, hair cracks on the surface of pieces seem to propagate in the direction of the piece axis and in the transverse direction, but in case of lower stress, cracks propagate diagonally in the direction of principal stresses, showing patterns socalled “cross-mark”.