Abstract: A systematic investigation into the fracture characteristics, contributing factors and fracture mechanism was carried out for fractured GH6783 superalloy bolts which had served for nearly 30000 h in a 1000 MW ultra supercritical unit. Techniques including macroscopic morphology observation, mechanical property test, thermal expansion measurement, metallographic analysis, X ray diffraction(XRD), scanning electron microscopy(SEM) and energy dispersive spectroscopy (EDS) were adopted. The results show that although the tensile properties, hardness of the bolt matrix still meet the standard requirements after long term high temperature service, its impact toughness and percentage reduction of area decrease significantly. The alloy exhibits insufficient surface oxidation resistance, and numerous pitting pits are formed at the thread roots. A gradient defect structure is generated in the pitting zone, consisting of an outer iron oxide layer, an intermediate multi alloy oxide layer and an inner degraded matrix layer containing acicular plate shaped brittle precipitates. The grain boundary β-NiAl phase coarsens and forms a continuous intergranular network, and hard brittle acicular Ni₅Al₃ phase precipitates inside the β phase, further aggravating material embrittlement. Low cycle alternating loads induced by unit start up and shut down promote fatigue cracks to preferentially initiate at pitting defects on thread roots and propagate inwards. Under the synergistic effect of surface oxidation induced fatigue cracking and SAGBO (stress accelerated grain boundary oxidation) embrittle-ment, the effective load bearing cross section of the bolt shrinks continuously, and macroscopic brittle fracture occurs eventually. This study provides theoretical basis and engineering reference for failure prevention and safety assessment of similar superalloy fasteners used in ultra supercritical units.