Abstract: The modes and causes of torsional fatigue failure of automobile engine crankshafts were analyzed by studying typical failure cases in torsional fatigue tests. The results show that the main torsional fatigue failure mode of crankshafts is rod journal oil hole failure, and abnormal failure modes are crank web failure and rod journal bottom center failure. The abnormal failure causes include the structure design, material defect, improper machining, excessive stress, and so on. Concave and convex of crank arm, raw material inclusions, internal machining marks of oil hole and excessive loading will lead to abnormal torsional failure of crankshafts. Finally, some improvement measures were put forward according to the failure causes.