In this study, the rotary bending fatigue test was carried out with low carbon steel(SM25C). The specimens were heat-treated in order to change the grain size, and investigated items are fatigue limit, small crack initiation, fatigue crack propagation behavior and possibility of fatigue life prediction according to the different grain size. The summarized result are as follows ; Fatigue limit of the smooth specimen was dependent upon the grain size. The fatigue crack initiation of the small grain size specimen was delayed more than that of the large grain size specimen. And the small cracks of small grain size specimen were distributed in the narrow region of the main crack circumference contrary to the large grain size specimen. The main crack was grown along the grain boundary having co-alliance with small cracks. The experiment material has quantitatively disclosed the possibility of fatigue life prediction because the fatigue crack propagation behavior is dependent upon the grain size.