Abstract: For the abnormal wall thickness reduction of transportation pipelines in service, a combination of testing methods of magnetic memory, acoustic emission, ultrasonic C-Scan and X-Ray inspection technologies was used to explore its cause. Conclusion was drawn that the defect was defined as metallurgical imperfections caused during smelting and rolling the tube(analogous to delamination flaws of tramp metal), which was not corrosion defects. The tubes could be serviceable under the condition of strengthening their monitoring timely.