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By means of creep properties measurement, microstructure and fracture morphology observation, the damage and fracture mechanism of a nickel-based single crystal superalloy was investigated during creep at moderate temperature. The results show that the deformation mechanism of the alloy in the latter stage of creep is that the primary-secondary slipping systems are alternatively activated, and the initiation of the micro-crack may be generated on the interface of the cubical γ′/γ phases in the intersection regions of two slip systems. As creep goes on, the micro-crack may be propagated along the γ′/γ phase interface which is perpendicular to stress axis, to form the square cleavage plane parallel to <110> directions. Thereinto, when the propagation of the cracks on (001) plane is intersected with {111} cleavage plane which is secondary activated, it may terminate the propagation of the crack. This results in the (001) cleavage plane possessing the square-like feature. Due to that the multi-cracks may propagate on different cross-section of the alloy during creep, and the tearing edge or secondary cleavage plane form along the bigger shearing stress direction at the crack tip, the multi-cracks are connected each other until the occurrence of creep fracture. It is thought to be the main reason of the uneven and multi-level cleavage characteristics.