The ratios of the number of cycles to slipband formation and to crack nucleation to the total life were determined experimentally in fatigue tests of smooth specimens of low-carbon steel with several grain sizes. The number of cycles spent in propagating a nucleated fatigue crack was predicted by an analysis based upon the results of a through-crack growth study. The change of crystal deformation with number of cycles was studied by the X-ray microbeam diffraction technique, and the characteristics of cyclically induced substructure were discussed in comparison with substructure due to monotonic deformation. A micromechanistic model for the fatigue limit was proposed to interpret the Petch-type relation between fatigue limit and grain size under the assumption of the critical value of microscopic stress intensity factor at the tip of cracks blocked by the grain boundary. From through-crack growth experiments at several values of the stress ratio R, it was found that the threshold stress intensity factor ΔΚ_th increased linearly with the square root of the grain size l for each R-value, and the relation between the effective value of ΔΚ_th and l was independent of R-value. In order to interpret these findings, a model for the threshold of crack growth was proposed under the assumption that the threshold condition was determined by whether the slipband near the crack tip propagated into an adjacent grain or not.