Abstract: Clamping bolt for camshaft timing pulley of an engine fractured during the durability bench test. By means of macroscopic examination, chemical composition analysis, hardness test, metallographic examination and fracture analysis, the fracture causes of bolt were analyzed. The results show that the fracture mode of bolt was fatigue fracture. The bolt was loose during using because of insufficient pretightening force,and cracks initiated on the screwed part of bolt under the action of alternating stress, which finally led to bolt fatigue fracture.