The failure of the condenser tube due to fatigue is caused in most cases by the vibration from neighbouring machines or repeated thermal expansion of the tube itself. The writers have conducted an experimental investigation recently to see the influence of the expansion in various degrees, annealing temperatures of tube ends and corrosion on the resistivity of the expanded condenser tube against fatigue. The results of the test are as given below : (1) The fatigue strength redrotion factor of an expanded condenser tube turned out to be about 25. (2) Condenser tubes annealed at 650°C and 750°C had smaller tensile strength and accordingly smaller fatigue strength when they were of unnotched type, but in case of an expanded tube specimen it showed nearly the same fatigue strength as compared with the one annealed at 550°C. (3) The variation in degree of expanding had scarcely any influence on the fatigue strength. (4) The failure due to fatigue initiated from the surface of the end part of the tube which is in contact with the head plate. This might be due to the fretting corrosion accelerated by minute relative slips between the outer and inner surfaces of the tube caused by expansion. Corrosion of the interior of the tube had little effect on the fatigue strength. (5) When the length of expanded part of tube is longer than that of the tube seat, the fillet radius at the edge of hole in head plate increases adding to the fatigue strength of the expanded tube.