The behavior of fatigue was studied by using low carbon steel bar with microhole(artificial small defect) under the condition of rotary bending stress which is made artificially at smooth surface and round notch root. The results obtained can be summerized; The behavior of non-propagating cracks which are produced at both tips of small defect occurred to dissymmetry, and it is found to be double size of small defect. For the range of l>lc, threshold stress intensity is constant. However, for the range of l<lc, it approaches to fatigue limit which artificial small defect is not contained. For the small defect within round notch root, its effect on fatigue limit is lower than the case of smooth surface, and it is no more effective than the round nothchis.