Abstract: According to Lamb theory, guided waves contain multiple symmetric and antisymmetric modes on a plate. When the wavelength is much greater than the plate thickness, classical plate theory can be used to understand the wave motion. There are two modes of propagation. One is called the extensional (S₀) and the other the flexural mode (A₀). In order to research these acoustic modes, a signal source named Fedical and the other named breaking pencil lead were used for a steel plate of 0.7mm thick. Both the fundamental extensional and flexural modes were detected with transducers. Acoustic modes were analyzed and wave velocities were measured. The experimental value of the extensional wave velocity is in close agreement with the theoretical velocity, but it is difficult to measure the flexural wave velocity accurately because of its dispersion. It can be concluded that extensional wave velocity is reliable for source localization in AE testing.