Abstract: Fracture accidents occurred on the second shaft of automobile gearbox during operation. The study of premature fracture failure of these shafts was made by means of SEM inspection, microstructure examination and chemical composition analysis. The result shows that the fracture has appeared fatigue feature, with initiation sites at the root grooves of the spline of the shafts. The network carbide and acicular martensites that existed in the microstructure of the root grooves of the spline due to improper carburating and quenching process of the shafts, led to the initiation and propagation of the intergranular microcracks and were the main reason that caused the early fracture of the shafts.