Abstract: Many defects in welds were detected by ultrasonic time-of-flight diffraction(TOFD) technique. The signal characteristics responded by the typical defects in weld were analyzed. The discrimination of defects by TOFD was described in detail and the results were compared with those by normal ultrasonic testing. It was showed that TOFD technique was superior to normal ultrasonic testing technique in intuitive display, abundant information and inspector-independent detection and accurate defect discrimination.