Abstract: The high cycle fatigue properties of a single crystal superalloy were investigated at 700 ℃ and 800 ℃. SEM and TEM were used to study the fracture surface and fracture mechanism. The results showed that the fatigue strength coefficient dropped, Basquin exponent rose and the fatigue limit decreased with the increasing test temperature. The fracture surfaces were the same at the above temperatures, each consisting of several 111 planes. The fracture mechanism was all quasi-cleavage fracture. Fatigue source zone, crack propagation zone and transient fracture zone could be seen on the fracture surface. The fatigue crack initiated at or near the surface and propagated along 111 planes. The river pattern, slip band, fatigue arc and fatigue striation could be seen in the crack propagation zone. The cleavage step and tear ridge could be seen in the transient fracture zone. The γ′ phase morphology still maintained cubic after fracture. Dislocations were unevenly distributed in the γ matrix of the alloy.