Abstract: Ultrasonic test is used for detecting welding spot of aluminum alloy. The ultrasonic echo features around the welding spot of aluminum alloy are analyzed. A program is designed by MATLAB, which provides two methods for measuring nugget diameter of welding spot of aluminum alloy. One is signal processing of A-scan contained in B-scan image, and the other one is image processing of B-scan. The calculated values of nugget diameter by the two methods are compared with the measured values by macrophotograph of samples. The results show that the ultrasonic method is available for measuring nugget diameter of welding spot of aluminum alloy. The first method is accurate but needs more data, and the second one needs a few data but the accuracy is not so good.