Abstract: A comprehensive analysis of microstructure, chemical composition, hardness, macro and micro features was carried out to study the fracture reasons of auto tensioner fastening bolts. The results show that excessive extrusion in the thread rolling process led to a lot of micro-cracks in the thread tip and processing defects at the root of threads respectively, which further expanded in the later heat treatment process, and caused the stress concentration and thus reduced the fatigue strength of the material when the bolts were in use. The premature formation of crack source led to fatigue fracture and failure of the bolts.