Abstract: Reasons of the formation of special longitudinal particle accumulations on the 440C materials discovered in MPI were investigated with fluorescent penetration test，metallographic examination and EDS analysis. The results showed that the particle accumulations were non-relevant indications, which were formed by absorbing magnetic particle because of the permeability difference of the carbide and matrix changed abnormally magnetic induction lines by the cause of the serious band segregation of the eutectic carbide in the materials.