Abstract: A low-frequency multi-mode ultrasonic Lamb wave method suitable for characterizing thickness, density and elastic stiffness tensors of the ultra-thin uni-direction fiber-reinforced composite layer 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 characterization method. In conjunction with the method of least squares, 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 all methods for different parameters are analyzed. Using the ultrasound coupler and a pair of broadband longitudinal wave transducers with 2 MHz center frequency, one can successfully estimate the parameters of the ultra-thin GF/PSF composite layer by combination with employing the leaky guided wave frequency-domain analysis method.