Abstract: A low-frequency multi-mode ultrasonic Lamb wave method suitable for characterizing elastic stiffness tensors, thickness and density of the ultra-thin transversely isotropic laminate composite is presented. The "ultra-thin" there means that the thickness of the plate is only a fraction of the ultrasound wavelength. The dispersion properties of the low frequency Lamb wave propagating along and parallel to the fiber direction are derived, which is the basis of the multi-mode Lamb wave characterization method. In conjunction with the method of least square, the secant algorithm is used to estimate the parameters of the ultra-thin fiber-reinforced composite layer. The reasons for evaluation error of the parameters and the sensitivity of the method to different parameters are analyzed. Using the ultrasound coupler and a pair of broadband longitudinal wave transducers with 2 MHz center frequency, we can successfully estimate the parameters of the ultra-thin grass fiber reinforced PES composite layer with by employing the leaky guided wave frequency-domain analysis method.