Abstract: According to the lately force magnetic theory, tiny Holl probe was used to practice magnetic flux leakage(MFL) testing on R3 middle-carbon steel. Experiments were proceeded under low and high MFL signals change after stress being applied. The significant effect of residual stress on MFL signals was pointd out, the relationship between residual stress and MFL signal was pointed out also. Experiments showed that it is possible to test the stress concentration with the above mentioned method.