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The cause of crankshaft fracture crack of a certain vehicle has been analyzed through macroscopic inspection, chemical composition analysis, microstructure analysis, hardness test, mechanical properties test and dimension inspection. The analysis results show that: All of its chemical composition, microstructure, hardness, mechanical properties and dimensions accord with the technical specifications. However, due to the bearing score of the seventh main journal and the wear of fillet, the crankshaft cracked at multi-points from the fillet. The reason is that the fracture of crankshaft is finally caused by large cyclic external force.