Abstract: The fracture pattern of the screw bolt on a marine gearbox had been analyzed by scanning electronic microscope (SEM). Simultaneously,based on the work condition and stress of the bolts,the forming mechanism and process of the fatigue cracks were analyzed. According to the analyses,the reasons that caused the bolts fracture were concluded and the corresponding measures were brought out,which had provided a guide for the safe operation of the machine and optimum structure.