The coaxing effect in plane bending fatigue was investigated on the plain specimens of annealed 0.5%C steel under zero and positive mean stresses.The main results obtained are as follows:(1) The mean stress has hardly any effect on crack initiation, but crack propagation is remarkably affected by the value of mean stress.(2) The Fatigue limit, in the case where the mean stress is positive, is controlled by the limiting condition for propagation of a micro-crack. This is closely related to the coaxing effect.(3) The Coaxing effect of a plain specimen, in the case where a non-propagating micro-crack formed under the first stress level is short, is more remarkable than that in the case where the micro-crack is long.(4) The coaxing effect of a plain specimen is bigger in the case of positive mean stress than in the case of zero mean stress.