Abstract: Three kind of defects in tank plate in 7 mm in thickness, the hole, conic pit and step pit, are comparably inspected by means of HTT-Ⅱ Flux Leakage Testing System. The different sensitivities to sensor of these defect signals resulted from the testing are quantitatively analyzed and the effective detection range of these defects are ascertained. The authors consider that the testing result is of good reference for the study on recognition of different defects.