Abstract: The reason of the magnetic traces on the different part of crankshaft fatigue testing specimen had been analyzed by means of macroscopic and microscopic examination. The results showed that the magnetic traces were caused by the banded structure,fatigue crack and grinding crack.The grinding cracks were caused by the prodigious grinding stress.The banded structure was very serious in the raw material,causing the uneven distribution of the quenching structure and residual stress,which accelerated the initiation of grinding crack.