Polyethylene (PE) pipes are widely used in transporting natural gas since 1960s. However, there are few published literatures on method for safety assessment of PE pipes. This paper introduces recent work in developing ultrasonic nondestructive testing (NDT) equipment and research on safety assessment of electrofusion (EF) joints of PE pipes. According to the geometrical characteristics and ultrasonic response, defects in EF joints can be divided into four categories, i.e., poor fusion interface, voids, structural deformity, and over welding. The defect of the poor fusion interface includes three typical forms, i.e., cold welding, unscraped oxide skin, and contamination of the fusion interface. Cold welding, which is the most common and dangerous defect, has been successfully detected by using ultrasonic NDT equipment and assessed with a patented cold welding Eigen-line method.