The deformation and damage mechanisms of a single crystal nickel-based superalloy containing 6.0%Re/5.0%Ru were studied through creep performance tests at 800 °C/860–880 MPa, microstructure and morphology observation, and dislocation configuration analyzation. It was found that, during the creep process at the intermediate temperature, the γ′ phase does not form a raft-like structure. After a creep fracture, the distortion degree of the cubic γ′ phase becomes greater when the observation region is closer to the fracture. The alloy has a long creep life at 800 °C, and the dislocation slipping or climbing in the γ matrix is the deformation mechanism at the early and middle creep stages. At the later creep stage, the γ′ phase is sheared by dislocations. Because of the low stacking-fault energy of the alloy, the superdislocation shearing into the γ′ phase can decompose on the {111} plane to form a (1/3) partial dislocation and stacking-fault configuration or cross-slip to the {100} plane to form the Kear–Wilsdorf (K-W) lock, which greatly improves the creep resistance of the alloy. At the later creep stage, the primary/secondary slip systems in the alloy are activated alternately, resulting in micro-cracks at the intersection of the two slip systems. As the creep progresses, the initiated cracks spread and propagate in the γ matrix phase along a direction normal to the stress axis and connect with each other until creep fracture occurs. This is the fracture mechanism of the alloy during creep at the medium temperature.