Abstract: The fracture cause of the non-quenched and tempered 48MnV steel crankshaft was analyzed by the methods of chemical composition detection, macroscopic and microstructure observation, fracture morphology analysis and mechanical properties analysis. The results show that the failure mode of the crankshaft was a typical fatigue fracture. The crack source was located at the transition fillet between the main journal and the crank pin. Too many indusions in the crankshaft steel were the main reason for the fracture of the crankshaft. A lots of sulfides were observed at the crack source area and the porous defects with the shape similar to the forging streamline were found on the crankshaft surface, leading to an easy generation of fatigue cracks. The cracks propagated along the strip sulfides and banded structures, and eventually caused the fatigue failure.