Abstract: Bolts of a certain type of automobile tyre were broken in service. Fracture analysis of those bolts was conducted by means of macro examination, fractography and chemical composition analysis. The results indicated that the fracture mode is the fatigue failure. The fatigue fracture occurred in the root of the screw where defects such as decarburization and cracks existed, and these defects made the zone to be a stress concentration area and caused the bolts to break finally.