To account for effects of nonlinearty on the wave-propagation characteristics, by using Green’s second identity a nonlinear consistent equation for water waves propagating over arbitrary depths is derived by introducing a function as approximation to the exact velocity protential function for the nonlinear goving equations, which can be simplified to teh linear uniform mild-slope equation given by Zhang and Edge[7] recently. In shallow water the equation reduces to a nonlinear equation of Boussinesq-type. In deep water the nonlinear dispersion relation for Stokes expansion is found.