Abstract: A bolt of 1Cr15Ni4Mo3N precipitation-hardening stainless steel was found to have a suspected crack during magnetic particle testing. However, the bolt wasn't found to have crack during the subsequent fluorescent testing. Metallographic examination was carried out. There were ribbon residual austenites in the matrix of the bolt, which destroyed the continuity of the microstructure and magnetic property, and thus resulted in magnetic mark display. The conclusion laid a foundation for magnetic mark judgment during magnetic particle testing in the manufacture process.