Rotating bending fatigue tests of plain specimens were carried out on HIP'ed SMF3035(density :7.83 g/cm3) and CIP'ed SMF3020 (density : 7.09 g/cm3) specimens, in order to investigate the effect of sintered density on the characteristics of the initiation and propagation of a crack through successive observations of the plastic replica method. The following main results were obtained. 1) σwo/σs of HIP'ed SMF3035 is 0.45 and CIP'ed SMF3020 is 0.39. 2) When σ> σwo, in HIP'ed SMF3035 a crack appears in an area having a finite size after some stress repetitions and in CIP'ed SMF3020 a crack appears at the edge of the defect just after stress repetitions. 3) When σ=σwo, nonpropagating microcracks are observed on plain specimens of both materials after N-1×107 under a metallurgical microscope. Therefore, the fatigue limit of plain specimens is determined by the limiting condition for propagation of a microcrack.