Abstract: The fastening bolt of GH6783 alloy for the main steam valve of a certain power plant broke during the reinstallation and hot tightening process. The causes of bolt fracture were analyzed using methods such as macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, metallographic examination and mechanical property testing. The results show that the bolt extended outward from the inner surface of the central hole, showing the characteristics of grain boundary oxidation brittle fracture. Improper β－aging treatment resulted in the absence of a narrow and continuous network at the grain boundaries β－NiAl phase, led to poor oxidation resistance of grain boundaries. During the bolt tightening process, the applied stress was higher than the grain boundary strength, ultimately led to bolt fracture.