Abstract: An investigation of dislocation structure and its impact on the mechanical behavior of a ?? strengthened nickel base single crystal alloy under high cycle fatigue (HCF) at 870℃ is studied. The results indicate that HCF lifetime reduced with increase of applied stress amplitude. In the early stages, the fatigue deformation occurs by forced bowing of dislocations through the narrow ? matrix channels on {111} planes. During mid-term fatigue stages, most of the dislocations formed in the matrix are located in the r?/ r"?? interfaces, their intersection and reaction produce new dislocation segments and three-dimensional dislocation networks. The Burgers vectors of the dislocations in the network are 1/2<110>. The interaction of cyclic stress with high temperature induced the precipitation of homogeneous globular r"?? precipitates, that is beneficial to fatigue strength. At the end of fatigue test, the cyclic stress led to the formation of persistent slip bands moving through the r? matrix channels and ?r‘? particles. Dislocation shearing through the r"?? phase was found occasionally. The extrinsic stacking faults was also observed in the present study.