Abstract: An automobile crankshaft fractured during the durability test. The fracture reasons of the crankshaft were studied by means of macroscopic observation, chemical composition analysis, scanning electron microscope analysis and metallographic examination. The results show that the fracture property of the crankshaft was fatigue fracture. The cracks originated from the surface of the transition R. The smaller size of the R and the deeper machining texture of the turning tool grain increased the stress concentration at the R. During operation, the cracks initiated at the R. Under the action of alternating load, the cracks propagated fatiguely and eventually led to the fracture of the crankshaft.