As material flaws have great influence on fatigue strength of cast or forged components, it is important to establish fatigue strength evaluation method of cast or forged materials with flaws. Previous study, fracture mechanics approach was made to estimate fatigue strength of cast steel with a round-shaped flaw, and“intrinsic crack model”was proposed. In this study, applicability of“intrinsic crack model”to estimation of fatigue life of cast or forged steel components with a small flaw is discussed, and following results are obtained. (1) Fatigue life of cast or forged steel with a small flaw can be estimated by fracture mechanics analysis regarding a small flaw as a crack of which length is the sum of a flaw length and an intrinsic crack one. (2) Fatigue life of a large forged component with non-metalic inclusions (MnS) can be estimated by fracture mechanics analysis using above “intrinsic crack model”.