Abstract: Polyethylene(PE) pipe was widely used in the gas distribution networks. Cold welding and over-heat welding could infect directly the mechanics performance of joint of PE pipes. It was discovered that there was an Eigen-line in the electro-fusion(EF) joint of PE pipes due to a great amount of separated micro-bubbles, and the distance between the line and heating wires could be used to measure the degree of welding. Furthermore, a unique method was developed to detect the degree of welding. A set of experiments showed that the method could be used in the research of welding procedure, and cold welding and over-heat welding quality inspection.