Abstract: The main bearing cap bolt of a natural gas bus engine was fractured. The causes of bolt fracture were analyzed through macroscopic observation, chemical composition analysis, scanning electron microscope and energy spectrum analysis, hardness test and metallographic inspection. The results showed that the failure mode of bolt was hydrogen embrittlement fracture. During service process, stress concentration occurred at fillet of bolt head under the action of a long period of stress, and there was an initial crack. The trace hydrogen in the bolt, water vapor and nature gas diffused and accumulated near the intial crack of the bolt. When the hydrogen content reached a critical value, the crack expanded, and finally the bolt was fractured.