Abstract: One of the connecting bolts of the top cover of a main propeller hub was found to be fractured during regular inspection. The fracture causes of the connecting bolt were analyzed by means of macro observation, micro analysis, metallographic examination, energy spectrum analysis, hardness test and stress analysis. The results show that the fracture property of the connecting bolt of the top cover of the main propeller hub was fatigue fracture. There was wear on the side of the connecting bolt of the top cover of the main propeller hub. The deflection angle of the fractured bolt increases, thus the bolt subjected to large additional bending moment. Which produced a large shear stress and superimposed the cyclic stress during use, resulted in fatigue fracture of the connecting bolt along the root.