Abstract: In the saturated steam with oxygen content less than 1 mg·L^-1 at 100℃, axial constant-amplitude force controlled fatigue tests were conducted on the precipitation hardening stainless steel Custom 450 with three stress ratios (-1, -0.6, 0.1). The fatigue fracture morphology was observed, and the ultra-high cycle fatigue fracture mechanism was investigated. The results show that the S-N curves of the tested steel presented a continuous decline shape without a horizontal asymptote, indicating that the tested steel had no fatigue limit. Three crack initiation patterns, namely forming crack sources at surface defects, internal inclusions and internal defects, were observed on the fracture surfaces. With the increase of stress ratio, the probability of the surface crack initiation increased while that of the internal crack initiation decreased.