Abstract: Suspected hairlines were often found in medium and low carbon high manganese steel in etching test of tower samples. The suspected hairlines were shallow in depth, and length of them often exceeded 30 mm, even through the whole step, which were different from traditional hairline defects. Formation causes of suspected hairlines were experimental studied by etching, magnetic particle inspection and metallographic examination. The results show that suspected hairlines of medium and low carbon high manganese steel were linear corrosion marks of banded structure produced in etching. Magnetic particle test can effectively prevent the misjudgment.