Abstract: Nonlinear effect of Lamb wave propagation in layered planar structures with weak interfaces has been studied in this paper. Under second-order perturbation approximation, the nonlinear effect of Lamb wave propagation may be treated as a second-order perturbation of linear response of Lamb wave propagation. The weak interfaces can be described by the interfacial spring model. According to the modal expansion approach for waveguide excitation, the second-harmonic fields of a fundamental Lamb wave propagation can be decomposed into a series of double frequency Lamb waves. It is found that changes in the properties of the weak interfaces will influence the expansion coefficients of double frequency Lamb waves in several aspects. Especially, changes in the phase velocities of Lamb waves caused by changes in the properties of the weak interfaces will remarkably affect the degree of cumulative growth effect of double frequency Lamb waves. The results obtained show that nonlinear effect of Lamb wave propagation can effectively be used to characterize the properties of weak interfaces in layered structures.