The behavior of the fatigue crack in a finite width plate and an infinite plate subjected to cyclic loading and corrosive action is examined theoretically. A crack propagation law is deduced assuming that a fatigue crack will extend when the damage accumulated at the tip of the crack reaches a critical value. It is found that the rate of crack growth is affected by specimen width and type of loading ' (constant load or constant net stress). This shows a good agreement with the test results by Munse, et al. It is also concluded that the estimation of corrosion fatigue strength of an infinite plate is possible if the factors, k₀ and q₀ in eq. (26), is determined from a corrosion fatigue test of finite width plate.