Structural members of ship in seaway are subjected to the repeated stress due to the hogging and sagging bending moment, or the difference of water pressure due to the change of draught. In rough sea, the impulsive forces due to the wave are added to the above repeated stress. Moreover, the ship is always in the environment of corrosion due to sea water. The combined action of these three factors-repeated stress, impulsive force and corrosion-may lead to failure of ship in stormy sea. Nevertheless, the strength of materials, such as steel or aluminium alloys, under such conditions has not yet been studied. In the present investigation, a simple method was used to obtain the effect of combined action of the above three factors on the strength of 60 kg/mm² high-tensile steel, mild steel and corrosion-resistant aluminum alloy. Fatigue testing machine of a rotating canti-lever type was used. The range of testing is restricted up to about N = 5×10⁵, as the number of repetitions of stress are 3. 5 per minute. Corrodant used is the artificial sea water which is dripped on the test section of the specimen. Impulsive force is applied on the point of loading of the rotating machine by a dropped hammer per 4, 10 or 30 repetitions of repeated stress. Impact energy can be adjusted by the change of the weight of hammer. The results obtained are as follows : (1) Considerable decrease in fatigue life was observed under the simultaneous action of intermittent impact and corrosion-fatigue compared with the corrosion-fatigue life. (2) The larger the impulsive energy (e. i. stress induced by impact) the shorter the fatigue life. In mild steel and aluminum alloy, the rate of decrease in life under an impulsive stress is nearly constant in spite of the change of repeated stress, but in the high-tensile steel, under a constant impulsive stress, the smaller the repeated stress, the larger the reduction in life.