Corrosion fatigue crack propagation of ship structural steel plates such as KA36 (TMCP) has been investigated from a viewpoint of hydrogen embrittlement. Hydrogen permeation coefficient was measured to be about 0.11 μA/cm in sour crude oil with H₂S and H₂O. It was simulated by cathodic hydrogen charging in some electrolytic solution. In this condition, the crack propagation rate of KA36 (TMCP) steel was accelerated as well as in sour crude oil. It increased with an increase in hydrogen permeation coefficient. It was well coincident with the spacing per cycle of brittle striation, which was observed in the accelerated crack propagation area in both conditions. The crack propagation rate, da/dN , increased with a decrease in the cycle of stress. Hydrogen content near the crack tip in the, ΔK of 37 MPa√m might be about one order of magnitude more than that in base material, which was estimated by both the observation of dislocation density in the plastic deformation region and the measurement of hydrogen diffusion coefficient of cold rolled steels. As the result, the hydrogen content in steel near the crack tip in high ΔK region might be approximately 0.1 mass ppm. Considering these results, it was concluded that the environmental enhancement of the crack propagation rate in sour crude oil was due to hydrogen embrittlement.