Fatigue crack propagation behavior of K40S cobalt-base superalloy under ambient atmosphere at 700 ℃ and 900 ℃ was investigated. The detailed fatigue crack propagation and fracture mechanism under the alternating loads were studied. The results show that, there is a defined threshold for K40S alloy at elevated temperatures. The fatigue threshold is 23.9 MPa·m 1/2 at 700 ℃ and 12 MPa·m 1/2 at 900 ℃. The significant decrease of the threshold with increasing temperature is associated with the oxidation induced embrittlement at crack tip. Observation on the fatigue fracture surfaces indicates a ductile fracture mechanism related to the fatigue crack growth. Fatigue crack propagation behavior of K40S cobalt-base superalloy under ambient atmosphere at 700 ° C and 900 ° C was investigated. The detailed fatigue crack propagation and fracture mechanism under the alternating load were studied. The results show that, there is a defined threshold for K40S alloy The fatigue temperature is 23.9 MPa · m 1/2 at 700 ° C and 12 MPa · m 1/2 at 900 ° C. The significant decrease of the threshold with increasing temperature is associated with the oxidation induced embrittlement at crack tip. Observation on the fatigue fracture surfaces indicates a ductile fracture mechanism related to the fatigue crack growth.