There have been no discussions concerning the effects of crude oil on the fatigue strength of ship hull structural members in large oil tankers because of the difficulty of conducting fatigue testing in crude oil containing H2S gas which is toxic to the human body. Accordingly, the authors manufactured sour crude oil corrosion fatigue testing apparatus for trials and then conducted corrosion fatigue crack propagation tests and corrosion fatigue life tests for KA 36 (TMCP) and KAS steel in sour crude oil containing 400 ppm of H2S gas. Consequently, it was found that corrosion fatigue cracks propagated on cleavage facets associated with brittle striation and corrosion fatigue crack propagation rates for the KA 36 (TMCP) and KAS steel are accelerated in the region where A K is relatively large in sour crude oil containing 400 ppm of H2S. The corrosion fatigue crack propagation mechanism is also discussed for these steels in a sour crude oil environment.