The technology of ultrasound can be used to evaluate properties of functionally graded materials(FGMs). Because of the inhomogeneity of the FGM properties, distribution function is used to describe the FGM elastic constant and density which are assumed to vary in the direction of the thickness. Taylor expansion method is used to solve Lamb wave propagation problems in FGMs, in which the distribution function is an arbitrary function. Phase velocity dispersion curves for Lamb waves in iron-based alumina FGMs are obtained by using an extension of the Legendre polynomial approach, and the effects of the gradient variation of iron-based alumina FGMs properties on Lamb wave propagation characteristics are discussed in detail. The conclusion could be useful for inversing elastic constant and density, which are varied along the thickness direction, of FGMs.