A new model that considers both the electrostatic and steric-hindrance effects based on the space-charge model and the steric-hindrance pore model has been proposed to evaluate the transport phenomena of an organic electrolyte across a nanofiltration membrane, The dependence of the membrane parameters, reflection coefficient and solute permeability on the electrostatic and steric-hindrance effects has been analyzed in the system of an organic electrolyte and a negatively charged nanofiltration membrane. This model has been verified in the system of a tracer organic electrolyte (sodium benzenesulfonate), a supporting salt (sodium chloride) and commercial nanofiltration membranes (Desal-5, NF-40, NTR7450 and G-20); the theoretical values of the membrane reflection coefficients agreed very well with the experimental results of them.