Generally, a membrane is a composite, made by forming a skin layer on a support. The ion permeability coefficient of the support is generally different from that of the skin layer. Therefore, the characteristic properties of the support also affect membrane performance. We theoretically discuss the effect of pore radius, porosity and thickness of the support on ion flux and membrane potential, keeping the parameters of the skin layer constant. It was found that it is impossible to neglect the effect of support even if the support has no charge and a high porosity. In some cases, the membrane property is mainly determined by the support.