Abstract: The experimental observations of nonlinear effects of Lamb waves have been reported. Firstly, the brief descriptions have been made for the nonlinear acoustic measurement system developed by Ritec. The detailed considerations of acoustic experiment system established for observing nonlinear effects of Lamb waves have been carried out. Especially, the analysis focuses on the time-domain responses of second harmonics of Lame waves by employing a straightforward model. Based on the existence condition of strong nonlinearities of Lamb waves, the wedge transducers are designed to generate and detect Lamb waves in aluminum plate. For the different space distance between the transmitting and receiving wedge transducers, the amplitudes of the fundamental and second-harmonic Lamb waves on the plate surfaces have been measured within a given frequency range. In the near range of a frequency where the fundamental and the double frequency Lamb waves have the same phase velocities, the quantitative relations of second-harmonic amplitudes on propagation distance have been analyzed. It is experimentally verified that the second harmonics of Lamb waves do have cumulative growth effects.