Fatigue strength of heat- and surface-treated steels is investigated by using equivalent stress ratios (R_<EQ>-ratios) which have been proposed as parameters for correspondence between fatigue test conditions of notched and unnotched specimens. Diagrams are exhibited by relations between R_<EQ>-ratios and notch-root-concentrated stress ranges Δσ_<NR> and characterized by two type of fatigue strength σ_<w1> a nd σ_<w2>. In usually annealed, normalized and tempered steels, fatigue strength transfers smoothly from σ_<w1>, to σ_<w2> lowering R_<EQ>-ratio. In severely heat- and surface-treated steels, the transition from σ_<w1> to σ_<w2> brings about a sharp decrease in fatigue strength. So, mechanisms of fatigue strength σ_<w1> and σ_<w2> are reconsidered in the present paper. As a result, it is concluded that fatigue strength σ_<w1> a nd σ_<w2> do not correspond to fatigue processes of stage I and II, but depends upon single-slip and double-slip mechanism related with morphology of fatigue slip bands, respectively.