Abstract: In the research, the method of charactering the type of weld defects is proposed, and image features which can be used to identify the type of defects can be obtained through comparing and analyzing on the imagery from different samples. Moreover, the characteristics of testing signal are analyzed to help identify defect types. The result indicates that the ultrasonic TOFD-D scan image is closely related to the geometry of the defect, and the extracted image features can be used to characterize the geometric shape of the defect, and then characterizing the defect's type. Combining with ultrasonic test signal, the types of common defects,such as crack, lack of penetration, incomplete fusion, slag inclusion, and porosity can be effectively identify through TOFD D scan imagery of defects.