More than 6.5% of the propeller shafts of Japanese ships classed to the Nippon Kaiji Kyokai (The Japanese Marine Corporation) have fractured, cracked, or badly corroded for these six years. It is considered that most of the above damages have been resulted from the corrosion fatigue caused by sea water corrosion combined with repeated stress. In torsional corrosion fatigue, to which this paper will be limited to refer, it is peculiar that number of small cracks, so-called“cross marks”or“star marks”which unexceptionally generate at 45 degree to the axis of shaft, appear at first on the surface of the shafts even under small stress, and rapidly develop into the fracture. In order to explain the behavior of fracture and also to determine the allowable torsipnal stress, the author has tested small finished steel specimens subjected to repeated torsional stress and the corrosion by drops of sea water and found that the fatigue limit does not almost exist for 10⁸ reversals and that such small cracks as“cross marks”found on the actual shafts also generate on the surface of the specimens. From the results of the above tests as well as the statistical analysis of the service data on the shafts, the author concludes the condition of the damage of the propeller shaft under sea water corrosion as follows :- (1) The fracture will occur in early stage of use, when torsional vibrating stress exceeds ±350kg/cm² (2) Cracks so-called“cross marks”may be found after considerable period of use at such locations of the shaft that concentration of stress exists when the stress is between ±350kg/cm² and ±100kg/cm². (3) Cracks will not occur, when the stress does not exceed ±100kg/cm². As it is difficult to limit the torsional vibrating stress of the shaft within ±100kg/cm² in all working range of revolutions, especially for medium and small internal combustion installation, the only recommendation to clear the damage of the propeller shaft is to protect it from sea water corrosion by good continuous brass sleeve liner.