It is important to obtain the information on propagation of fatigue crack in ship hull structures for the prediction of service lives of structures subjected to alternating load in a corrosive environment. The effect of sea water on the fatigue crack propagation was examined for mild steel by using the center notched plate specimen and the slotted structural model. It was found that the correlation between the range of stress intensity factor Δ K and crack propagation rate da / dN for mild steel in sea water could be given in the same manner as the Paris's law, that is, da / dN = c (Δ K ) ^m, where m is nearly equal to 3. 5 both in the environments of sea water and air, and C is a coefficient which depends on the environment and load frequency f . In the sea-water environment, C is affected remarkably by the frequency but slightly in the air, and it could be given by the following equation. C =- A log f + B where A and B are coefficients which depend on the environment. Correlations between the stress intensity factor and the crack propagation in the root of web stiffener and in the web plate of structural model were in good agreement with those in the center-notched plate specimen. Finally, using the above mentioned corrosion-fatigue crack propagation law, fatigue crack propagation lives were calculated for the root of web stiffener and the web plate at the slot corner in the transverse web frame ring of the actual ship.