Abstract: Abnormal sound suddenly occurred in the high horsepower engine while a car was running. After opening the engine, it was found that the crankshaft had cracked at the fifth cylinder and the bearing had damaged and locked. In order to find the failure cause, macro trace observation, fracture analysis, metallographic analysis, chemical analysis, and hardness measurement were carried out. The results show that the crankshaft first cracked, leading to sharp edge on the surface, and then resulting in the damage and locking of the bearing. The crack of the crankshaft is torsion fatigue crack initiating from the oil duct wall (under the hardening layer). The crack was mainly caused by abnormal torsional stress. There were lots of MnS nonmetallic inclusions in the non-quenched steel matrix, which promoted the initiation of the fatigue crack. Then tests showed that the torsional damper had failed.