Authors have been studied to estimate the effect of flaws on the fatigue strength of castings or forgings quantitatively. In our second report, the effect of a small flaw, such as a non-metallic inclusion, was discussed by fracture mechanics analysis using “intrinsic crack model”. In this report, the effects of forging flow direction and non-metallic inclusion on fatigue strength of forged steel were discussed. Two types of JIS SCM440 steel, which have different index of cleanliness, were applied to fatigue tests. Fracture mechanics analysis using “intrinsic crack model” was made to the results of fatigue tests to clarify the effect of forging flow direction and non-metallic inclusion. The following results were obtained. (1) The anisotropy of fatigue strength of high strength low alloy steels due to the forging flow direction was mostly affected by the non-metallic inclusion size which was elongated parallel to the forging flow. (2) The fatigue strength in the direction normal to the forging flow can be estimated by fracture mechanics analysis regarding a non-metallic inclusion as a crack. In this analysis, “intrinsic crack model” was applied and the fatigue limit of the specimen parallel to the forging flow direction was used. (3) A component, which has the forging flow normal to surface at a high stress concentration point such as the fillet of crankshaft, shows low fatigue strength. The most effective countermeasure in such a case is to improve the forging design to change the forging flow direction at the high stress concentration point. Use of the high cleanliness steel, in which the size of non-metallic inclusions are decreased, also improves the fatigue strength of such a component, and can be one of practical solutions.