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Stepwise S-N curves were obtained under rotating bending fatigue tests atelevated temperatures for an 1Cr-0.5Mo low alloy steel, as well as fatigue tests at roomtemperature for surface hardened specimens by shot peening or carburizing treatment. Inthe high cycle region, internal fracture with fish-eye pattern was dominant. Why internalfracture does occur at elevated temperatures for specimens of the low alloy steel withoutany surface hardening treatments? It has been pointed out that the existence of oxidizedsurface layer plays an important role to prevent cracks from initiating at the specimensurface. But any evidence verifying the effect of oxidized surface layer on fatigue behaviorhas not been obtained. The steel used was hardened remarkably under cyclic stressing atintermediate temperatures due to dynamic strain aging. It is considered to be difficult forStage crack to initiate at specimen surface, because the slip bands hardens graduallyduring fatigue test at elevated temperature. This process has to bring the same effect assurface hardening treatments of shot peening and carburizing on fatigue fracture behavior.