Abstract: The second harmonics of Lamb wave retain the characteristics of two- dimensional spatial accumulation once Lamb wave phase velocity equals the longitudinal one of solid plate material. For this case, analytical expressions of the second harmonics are presented in the present paper, and quantitative computations are performed for some Lamb wave modes to describle the nonlinear acoustic propagations in the plate. It is also found that the second harmonics of A₀ and S₀ modes must not have characteristics of spatial accumulation.