Abstract: A marine diesel engine crankshaft fractured after running for less than 3 a. The failure analysis was conducted on the crankshaft by mechanical property test, chemical composition analysis, metallographic analysis, fracture observation and finite element analysis. The results show that the fracture property of the crankshaft was fatigue fracture under low stresses. The protective slag involved in the electroslag casting process was the direct reason for fracture of the crankshaft. The substandard tempering and nitriding treatment further reduced the fatigue strength of the crankshaft, and accelerated the propagation of the fatigue cracks, eventually leading to the fracture of the crankshaft.