Abstract: The inner cracks were found in the motorcycle shafter after drilling,and the causes of forming cracks were analyzed by fracture observation,metallographic examination,hardness testing and numerical simulation.The results show that the reasons for spline shaft forming cracks is due to the nonmetallic inclusions in the raw material,the center of blank endure higher tensile stress because of the unreasonable drawing process,and the cracks appear when the tensile stress exceeds the tensile strength of the material.The cracking of the spline shaft is brittle cracking.It is suggest to check the content of the inclusions in the raw materials,and it is also proposed that pickling and phosphate should be carry out before drawing in order to avoid the cracks.