Corrosion-fatigue and prior stressless corrosion fatigue properties in sea water of mild steel and its welded joint were investigated by the rotating cantilever machines and the general conclusions obtained are as follows : (1) The effect of time of prior stressless corrosion for the base metal and welded joint are remarkable at the beginning of the corrosion and after three months immersion the fatigue strength of each material decreases to about 80% of the fatigue limit in the air and hereafter the effect of time of corrosion becomes small. The deterioration of the strength is probably due to the presence of corrosion pits. (2) In the prior stressless corrosion, the degree of corrosion of deposite metal is not.always severe compared with that of base metal but in some cases the latter is rather seriously corroded. (3) In the corrosion-fatigue, owing to the combined action of corrosion and repeated stress, the corrosion pits will be rapidly formed which may be due to the potential difference between the tensile and compressive sides.. Fatigue cracks start from corrosion pits and spread over a section. The strength decreases markedly and the fatigue limit is not obtained, At reversals of 3 × 10⁷, the fatigue strengths for base metal and welded joint are 6.5 and 7.3 kg/mm², which are about 30 and 40% of the fatigue limits in air, respectively. (4) Fatigue limit for welded joint in air is influenced by the boundary portion of the base and deposite metals, and is only about 80% of that for base metal. But in corrosion-fatigue, the effect due to boundary portion is covered with the combined action of the corrosion and stress reversals and the fatigue strengths of both metals are nearly equal.