The effect of carbon content on fatigue crack initiation behavior has been investigated by the plastic replica method using four kinds of carbon steels with the same size of ferritic grains. The fatigue limit of each of these materials has been related to the existing limit of the nonpropagating cracks. Therefore, the purpose of this study is mainly to examine the effect of carbon content on the existence of nonpropagating cracks. From the experimental results, it appears that the nonpropagating cracks in low-carbon steels (S 15 C and S 25 C) were almost blocked by the boundaries of ferritic grains. On the other hand, nonpropagating cracks in medium-carbon steels (S 35 C and S 45 C) tended to be blocked by pearlite. Though it is well known that fatigue limit is proportional to the mean grain size according to the Hall-Petch equation, that of these materials of the same ferritic grain size was strongly dependent on carbon content.