Abstract: Polyethylene (PE)pipe is widely used in the gas distribution networks.Cold welding caused by insufficient input of heat energy, which can not be detected by nondestructive test methods available, is one of the most dangerous defects in the joint of PE pipes.It was discovered that there is 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 can be used to measure the degree of cold welding.Furthermore, a unique method was developed to detect cold welding.The degree of cold welding can be assessed with an empirical formula by measuring the distance between the line and metal wires with a proprietary system of ultrasonic inspection.Experiments show that the method can accurately detect cold welding and be used in the research of welding procedure, and quality inspection.