Abstract: This paper analyzes the thickness-twist wave propagating in a functionally graded piezoelectric bi-material plate. The upper and lower surfaces of the plate are mechanically free and electrically open. The material properties of the plate vary exponentially along the plate thickness. The electro-elastic fields satisfying the governing equations and the boundary conditions are firstly derived. Then the interfacial continuous conditions are utilized to obtain the relations that should be satisfied by thickness-twist waves. A numerical example is provided to show the effect of the material inhomogeneity on propagation characteristics of thickness-twist waves. The obtained results may be useful for the application of functionally graded piezoelectric materials to acoustic wave devices.