Abstract: Girth weld cracking is one of the main failure forms of buried oil pipelines and it threatens the safe operation of the pipelines. At present, it is one of the difficulties for the inline inspection (ILI) companies at home and abroad to identify and quantify girth weld defects accurately and establish a unified ILI precision requirement. One of the feasible methods to solve this difficulty is to develop the ILI technique suitable for the plane defect of girth weld based on the existing ultrasonic ILI technique which is suitable for axial crack. In this paper, the ultrasonic ILI device was developed. Then, based on multiple full-size traction test and actual pipeline inspection results, the ultrasonic inspection & identification and size quantification model of girth weld defect was established. Finally, the magnetic flux leakage inspection results were compared with the field excavation results of girth weld defects to verify the detection accuracy of the ultrasonic ILI device. It is shown that the serious plane defect of girth weld which is missed during magnetic flux leakage inspection can be identified by means of ultrasonic ILI, but the defect identification accuracy and the size quantification model shall be improved further based on more excavation validation data. The establishment of defect identification and quantification model can provide the reference for the application and development of ultrasonic ILI technology.