Abstract: Fracture analysis of a high-strength bolt in laying head is conducted by a variety of means. The results show that a fragile fatigue fracture happened in the bolt. Surface defects in the bottom of screw threads induce a very high degree of stress concentration due to improper mechanical processing,and full-decarbonization around the defects during the following heat treatment reduces fatigue strength of the bolt greatly,thus resulting in the early fracture of the bolt.