Abstract: The defects in Al-alloy thin plate were studied, especially the cause of delamination and its distribution. After discussing the merits and demerits of some nondestructive testing methods about metal thin plate, ultrasonic transmission method was chosen to test Al-alloy plate. Some manual defects which had different diameter were made to simulate delamination defects. The amplitude of transmission signal was processed to plot calibration curve. The delamination defects were easy to distinguish and the testing sensitivity was high because the ultrasonic beam was perpendicular to the defect surface.