Based on the Nernst-Planck equations of ion flow, a simple model was proposed for the current-voltage characteristics of an ion-exchange membrane under limiting current density. The numerical analysis depicts the influence of parameters such as solution concentration, diffusion boundary layer thickness, diffusion coefficient and transport number in solution and in membranes on current-voltage curves, as well as the profiles of the concentration and potential in membrane phase and in diffusion boundary layers. Limiting current density is discussed with the parameters and is consistent with the well known expressions. This work can be regarded as a supplement of the previous studies by Spiegler.