Abstract: Inconel783 alloy bolts in the medium-pressure valve of an ultra-supercritical unit fractured during service under high temperature and high pressure conditions. The cause of fracture was studied by macroscopic inspection, microstructure analysis, mechanical property test and phase analysis. The results show that the bolts fractured in a stress-accelerated grain boundary oxidation brittle fracture mode. Improper heat treatment of the bolt resulted in incomplete austenite grain boundaries without continuous β-AlNi phase. When the bolt was used under high temperature and high stress conditions, oxygen diffused along the grain boundary rapidly under stresses, resulting in the embrittlement of grain boundaries; therefore the intergranular cracks were initiated. The cracks propagated, and eventually led to the fracture of the bolt.