Abstract: A connecting rod bolt of an engine fractured during the durability bench test of the engine. The fracture reasons of the bolt were analyzed through macroscopic inspection, chemical composition analysis, scanning electron microscopy analysis, metallographic inspection, energy spectrum analysis and so on. The results show that:the fracture mode of the connecting rod bolt was multi-source fatigue fracture; there were plenty of elements of phosphorus and zinc inside cracks, which indicated that the cracks had been produced in thread rolling process; the phosphatizing liquid infiltrated into the micro cracks during the following phosphatizing treatment, and the fatigue cracks gradually expanded and led to the fracture of the bolt during the durability bench test.