Abstract: A detailed fractographic analysis was performed on a nickel-based superalloy under complex loading with superimposed compressive peak overload at 700℃. The fatigue cracks were observed to be presented in semicircular striations with relatively wide spacing. The measured fatigue crack growth rate da/dN from the fractography was found to be related to ΔK in the way of paris formula. It also indicated that fatigue cracks were not propagated continuously by extending a long crack but by a successive process of "stimunated nucleation at the front of crack tip" and "jump connection" with the main crack. Under the effect of the superimposed Sp there were three different types of "step" morphologies observed on the fracture surface, a crystallographic orientation relationship in the morphologies were proposed. The mixed fatigue and creep morphology reflected the influence of their interaction at high temperature.