Nondestructive evaluation tests for ferromagnetic structural materials were performed by magnetic flux leakage technique using a fluxgate sensor. For an artificial defect of 0.3 mm width, changes in perpendicular magnetic flux densities B_z could be measured at 5 mm lift-off height and changes in longitudinal magnetic flux densities By could be measured at 10 mm lift-off height. For artificial semi-circle and triangle surface defects, the distribution of magnetic flux densities corresponded to the cross section shapes of these defects. In-situ magnetic flux measurements B_y for permalloy under tensile loading were also performed. And changes in B_y were related to the tensile stress, where B_y was found directly proportional to stress even at elastic region.