In order to elucidate the essential structure of S-N curve, this paper discusses a prediction method for fatigue life and fatigue limit of materials containing various defects and the nature of scatter of fatigue strength influenced by presence of defects. All commercial materials contain some defects. Although graphite nodules are not inteded as a material defect from the viewpoint of metallurgy, they behave like a material defect from the viewpoint of mechanics of fatigue. Nonmetallic inclusions in bearing steels, spring steels, tool steels, etc., are a typical defect both from metallurgy and mechanics of fatigue. Defects and surface roughness in additively manufactured (AM) materials have serious influences on fatigue strength and quality control for realizing the application of AM to power components. This paper shows an extended application of the √area parameter model to not only fatigue limit but also to fatigue life in S-N data of various steels and AM materials in terms of scatter and quality control. The application of the proposed method to fatigue damage concept under variable amplitude loadings will be suggested.