Abstract: In the acousto-ultrasonic technique,a broadband ultrasonic wave is injected onto the surface at one location of the tested plate with the help of a longitudinal transducer and a receiving transducer is coupled to the same surface at another location.Thus,understanding the propagation characteristics of the ultrasonic wave is essential for successful application of the technique.In this paper,it has been found that the dominant acousto-ultrasonic waves produced experimentally in thin plate were multimode Lamb waves.The broadband generation and detection of Lamb waves has been performed in two aluminium plates with defferent thickness using the acousto-ultrasonic method.