Fatigue tests were carried out on mild steel plate specimens with various lengths of cracks. The relation between the alternating fatigue limit σ_w and the crack length was well expressed by following equation. R_p(c)w = αe(sec πσw/2σy - 1 ) = const. where, R_p(c)w is the cyclic plastic zone size at fatigue limit, αe is the equivalent crack length of an infinite plate, and σy is the yield strength of the material. The size of the persistent slip band zone on the fatigue limit is about one tenth of R_p(c)w and is nearly equal to the measured mean grain size of the material. R_p(c)w has close relations with the crack length corresponding to the fatigue limit of a plain specimen, the material constant which was already obtained in the criteria of fatigue crack initiation at notch roots, and the critical notch root radius at the branch point.