Corrosion fatigue tests of the three types of specimen (small size elementary component, middle size elementary component and structural component) with boxing fillet welded joint of TMCP steel (KA 32) were conducted in synthetic sea water at 40°C which is nearly the same environments as actual ballast tanks. As a result of testing and fatigue life prediction analysis, the following conclusions were obtained. (1) Corrosion fatigue life in 40°C sea water shows 50% reduction comparing with the life in air (corrosion factor for strength ( K_c ) is 1.3). This environment factor is the same for three configurations of specimen. (2) Fatigue life at failure of small size specimen is equal to the number of cycles in which the crack in structural component specimen propagates to 30 mm length. This phenomenon is shown in both air and 40°C sea water. (3) Fatigue life prediction of ship hull structure is simplified by adopting the proposed evaluation methods that replace the complicated small crack growth analysis at the toe area of welded joint by the failure life in small size specimen.