Abstract: Three half-shafts of a heavy truck fractured after running for hundreds of kilometers. The failure mode and cause were analyzed by macro and micro observation, metallographic examination, hardness testing and finite element simulation. The results showed that the root of the half-shafts firstly presented high stress torsional fatigue cracking and finally torsional overload fracture happened along the semi axis cross direction. Repeated large torque was the fundamental reason for fatigue fracture of half shafts. Stress concentration caused by the machining steps at the bottom of the spline groove and plenty of non-metallic inclusions on longitudinal direction further reduced fatigue resistance. Enhancing strength level and metallurgical quality, optimizing chassis design and spline groove structure should be implemented to avoid such failure.