Abstract: The fracture behaviour of mechanical gear shaft of the speed changing box of a certain type was analyzed. The microstructure and mechanical properties of the gear shaft were investigated. The results show that the matrix organization of automobile gear shaft is tempered martensite, and there are no air holes or other metallurgical defects in the root and matrix of the gear shaft; fatigue cracks begin in the stress concentrated thread root of the gear shaft, and after a short-term fatigue growth, the cracks develop horizontally, and finally split lengthwise. The poor mechanical properties of the gear shaft and the high circulating working stress are significant reasons for the cracking of the gear shaft.