Abstract: The magnetic trace phenomenon in nondestructive detection of 48MnV micro-alloy metal automobile engine crank in manufacturing process was analyzed. It indicated that the magnetic trace was because of raw material ununiformity deformation in forging which caused raw centre segregations flowed to the inner subsurface of crankpin and uncovered after by machine process. The metallographic characteristic was described and it proved that the essential of the magnetic trace was raw carbon centre segregations distributed in subsurface of the part. The influence of magnetic trace on fatigue properties were researched. It indicated that raw centre segregations on the part which its raw material passed by received inspection had no influence to the fatigue strength of crank and had not become the prior initiation of fatigue crack, in other word, the centre segregations area had the same behavior with the normal area in the loading process.