Abstract: NoPig technique is a new trenchless inspection technique developed by N.P. Inspection Company. It is suitable for the in-service defectinspection for the steel pipelines transporting oil, gas and water. The pipe diameter ranges from 76.2 mm to 914.4 mm. The basic principle of NoPig technique is that multi-harmonic current is activated onto the proposed both ends of pipe to measure the upward magnetic field of pipe, and further to display the frequency spectrums of the current and magnetic field. The ratio between these two spectrums are evaluated to finally recognize the changing range of pipe wall. Currently the technique is extensively applied in trenchless inspection for oil & gas transmission pipeline. Eon/Ruhrgas and VNG Pipeline Operation Company, Germany, RAG Company, Australia and Applus RTD Company have already performed their applied study. Research shows that it is suitable for non-piggable pipelines and bears high axial positioning accuracy. NoPig technique makes pipeline inspection easier, reducing inspection cost, owning a broad future.