Abstract: This paper presents a fundamental way of using ultrasonic TOFD (time of flight diffraction) method to detect, locate and quantify the flaws in the weld. The ultrasonic TOFD signal characteristics and diffraction regularities are studied by testing thick aluminum alloy plate, and the optimal technological parameters for test and measurement accuracy are discussed too. The experiment results show that the measured dimensions of the defects accord with the real ones, and the study provides the foundation for nondestructive testing of practical weld of thick aluminum alloy plate.