Abstract: Two stud bolts used for high pressure outer casing faying face of turbine in a power plant fractured when installing. Its material was 20Cr1Mo1VTiB steel. The fracture reason of the bolts was analyzed by means of fracture analysis, chemical composition analysis, metallographic examination and mechanical properties testing. The results showed that there were serious microstructure defects in the bolt material, and its mechanical property index didn′t conform to technical requirements. So bearing capacity of the bolt was weakened. Stress concentration effect at the thread position aggravated notch sensitivity of bolt, then it led to brittle fracture happened at the thread position.