Twelve transmission shafts were developed unilateralism torsion fatigue tests. Macroscopic and microscopic fracture features of transmission shafts were observed and analyzed. No defect was found in the origin which was usually located at the bolt bores or nearby the bolt bores because of the stress concentration. Using the fractography quantitative analysis technology the propagation life, the origination life and the relation curve between the crack length and the cycle numbers for all of the twelve shafts were estimated. For only one origin on the fracture surface the total life of the shaft with two expanding directions is usually obvious longer than that with one expanding direction, but the main propagation life is obvious smaller. However for two origins and two expanding directions on the fracture surface the total life of the shaft is usually shorter.