The authors investigated into the causes of failures frequently occurred in propeller shafts in way of the big end of cone part, using the results of a series of rotary bending fatigue tests with large specimems carried out by the Research Laboratory of the Nippon Kaiji Kyokai. According to the results of investigations, the failure of shafts is assumed to occur in such a manner that micro-cracks produce in these parts of the shaft at an early stage under low stress reversals, and the cracks so produced propagate due to the excessive stress of the shaft when the ship is in rough sea, causing the failure of the shaft. The results of a careful examination of the relationship between the shell patterns on cracked surface and the roughness of sea indicate that the propagation speed of cracks is in the order of 0.050.08μ/cycle and the stress which shaft is subjected to is considered to be±1214kg/mm², which implies that there is a large possibility of failure if microcracks have produced. Accordingly, the authors would like to emphasize that utmost caution should be paid to the watertightness and workmanship of press-fitting the propeller at the big end of cone part so as to prevent from the initiation of miro-cracks, and further that in rough seas, ships should be handled with care so that the stress produced in the shaft may be as small as possible.