Abstract: The reason of failure and break had been investigated for heavy diesel engine crankshaft which broke by fatigue on the basis of the physical test and chemical analysis,macrograph and microfracturegraphy (SEM、EDS) of the broke crankshaft.The results show that the initial fatigue crack appears near the surface defect of the oil hole of the crankshaft inner.The defect is lager (about 767 μm×500 μm ) and caused by cutting tool tip break apart during machining.The fatigue crack propagates with a repeated-turning stress and finally the crankshaft broke at early stage.