Abstract: The gear shaft of an excavator fractured. The causes of the fracture of the gear shaft were analyzed by means of macroscopic observation, chemical composition analysis, hardness test, scanning electron microscope analysis and metallographic examination. The results show that the radius of the gear shaft did not meet the technical requirements, the radius arc was not smooth, and the depth of the carburized layer was not qualified, so that the stress concentration at the initiation of micro cracks, further fatigue propagation of microcracks occurred, eventually leading to the fracture of the gear shaft.