Abstract: The high cycle tension-tension fatigue tests (stress ratio was 0.1)of smooth and triangular notched samples of nickle-based superalloy M951 were performed at 700 ℃ and 900 ℃, and the fracure morphology were analysed. The results show that M951 alloy had superior high cycle fatigue properties at elevated temperatures. At 700 ℃, the fatigue limit of smooth and notched samples were 320, 294 MPa, respectively, and the alloy exhibited very slow notch sensitivity at the temperature. At 900 ℃, the fatigue limit of smooth sample was 315 MPa and the alloy exhibited no notch sensitivity. The fatigue cracks of the smooth samples originated from two sites, solidification defects and a Y-riched phase, 2/3 of the failures resulted from the front and the rest fatigue failure from the Y-rich texture. The fatigue fracture of the notch samples were obvious multi-cracks and the main causes of cracks forming was stress concentration.