In order to clarify notch effect on fatigue strength of an austenitic stainless steel, rotating bending fatigue tests were carried out for smooth and notched specimens at 573K. Clear endurance limits were recognized and the fatigue strength did not decrease continuously with increasing stress concentration factor Kt, notwithstanding any non-propagating crack was not recognized at notch roots for specimens endured at 108 stress cycles. According to hardness tests, an area of notch root hardened during fatigue test by dynamic strain aging. The hardening behavior occurred remarkably with increasing Kt. Effects of Kt and the hardening behavior on fatigue strength canceled each other, and then fatigue strength became insensitive to Kt. Fatigue strength σSIT obtained by stress increment fatigue tests became higher than the ordinary fatigue strength for each specimen of Kt, that is, the coaxing effect was recognized clearly. This result indicated that the strength for fatigue fracture increased gradually during fatigue test. σSIT had a peak value at Kt of 2.0 and it decreased with increasing Kt sensitively. The increasing behavior of strength for fatigue fracture depended on Kt and fatigue fracture was controlled by circumstances of hardened material and stress amplitude considering Kt.