The effect of corrosion on the propagation rate of a fatigue crack was studied to get the relation between an applied stress and the corresponding fatigue life. From the theoretical analysis it was found that the corrosion fatigue, even if there is no effect of creep or strain rate, is strongly time dependent. Rotating bending tests on the high tensile steels were carried out at the frequencies of stress of 140, 20.5 and 3.5 cpm, to compare the theoretical analysis and the following conclusions were obtained. 1. The crack propagation rate in sea water is larger than that in air, and dc/dN would be expressed by eq. (7). 2. The propagation of a fatigue crack is affected by the frequency of stress and the propagation factor in air, k ₀ increases with the decrease in frequency. On the other hand, the propagation factor in sea water changes as k ₀exp. ( q ₀ N /ω) and consequently the effect of the frequency is more remarkable.