and vacuum environments. The fatigued specimens were metallographically examined by means of optical and scanning electron microscopies and electron probe microanalysis. The following conclusions were obtained: (2) The difference of fatigue lives in air and in vacuum environments tended to become smaller for the specimen with coarser grains. (3) The environmental dependence of fatigue life can be explained based on the property of oxide scale, decarburization and internal oxidation behaviors, and the crack propagation rate.