Abstract: The principle of Tri-axial magnetic flux leakage(MFL)pipeline in-line inspection(ILI)technology is introduced.The leakage field is a three dimensional magnetic vector,and the MFL tool records MFL signal using individual sensors in all three directions:axial,radial,and circumferential.The signal characteristic recorded by Tri-axial sensors is analyzed in detail:according to the axial MFL signal,the estimated defect width is unreliable,and defect length and depth can be defined,but it can not be solely relied upon for high accuracy results.Defect length can be defined crisply from the radial MFL signal,and defect depth also can be determined approximately when combining with the axial MFL signal,but defect width is still difficult to accurately define.Defect width and length can be determined from the circumferential MFL signal with high definition and repeatability,so it increases accuracy in depth estimation when combining with the MFL signals of the axial and radial.Research and field applications show that Tri-axial MFL ILI tool not only can detect various usual metal loss defects,but also can identify unusual defects,such as narrow axial defects,girth weld defects,spiral weld defects,and dents,which are difficult to detect by traditional MFL ILI tool.Comparing with traditional MFL tool.Tri-axial MFL tool can considerably improve the sizing accuracy of defects.