Abstract: A Japanese scholar,Hashimoto,proposed using the effective permittivity of grating and the discrete Green function to investigate the properties of surface acoustic wave propagation in periodic metal gratings with the finite element method,and developed a program compiled by Fortran,which has been used to design the surface acoustic wave devices by many international companies.This paper gives the results of Hashimoto program with an example of Rayleigh waves propagation in aluminium grating on 128°YX-LiNbO₃ substrate.The frequencies of the upper and lower stopband edges are read out and taken into the expression of the dispersion relation of COM theory.The figure drawn by this expression is the wavenumber dispersion curve to obtain COM parameters.