This research experimentally investigates the symmetry properties of the second harmonic Lamb wave. One anti-symmetric Lame mode pair (a2-a4) and one symmetric mode pair (s1-s2) are evaluated under the same experimental conditions. The wedge method is used to generate and detect a specific Lamb wave mode and the Morlet wavelet is employed to extract amplitudes of the fundamental and second harmonics. The results show that only symmetric second harmonic Lamb modes can cumulatively grow with propagation distance.