Abstract: The variation of magnetic memory signals of 18CrNi4A notched specimen was studied under fatigue loading.The research shows that the magnetic memory signals varied regularly with the increase of cycle number in the stress concentration position of the specimen.And in the test process,the Hp(y) curve had a zero value when the specimen was loaded in some specific cycle number.And the zero value hardly changed in the succedent test process until the specimen was fractured.Magnetic memory testing is comparatively accurate to judge the damage state of 18CrNi4A case-hardening steel.