Abstract: In783 alloy bolts used in reheat valve of ultra supercritical steam turbine fractured frequently, and the fracture forms were various. In order to study the influencing factors of the fracture behavior, the rupture fracture test of the bolt material was carried out under different stresses and temperatures, and the fracture samples were examined by metallographic examination and fracture analysis. The results show that the aging brittleness of the bolts was greater at 650℃ and 600℃, and its rupture plasticity was very low. The fracture mode of the samples were mainly intergranular brittle fracture. With the extension of the fracture time, the oxidation characteristics of the crack source and the corrosion characteristics of the fracture surface were more and more obvious, and the fracture mode gradually changed to intergranular oxidation fracture.