This paper presents a modified Donnan-Steric-Pore model (DSPM) to predict the rejection of mixture of NaCl:Na2SO4 in nanofiltration (NF) membrane based on the extended Nernst-Planck equation with the incorporation of charge-and steric effects for the transport of ions inside the membrane and incorporation of concentration polarization effect for mixture of ions/solutes. With this approach, the permeate flux can be calculated based on the concentration of ions at the membrane surface. The membrane performance was modelled using three parameters namely; effective pore radius, r(p), effective ratio of membrane thickness to porosity, DeltaX/A(k), and the effective charge density, X-d. The simulation results shows that the model can predict the tendencies and patterns of rejection and flux reduction behaviour reasonably well for system containing NaCl:Na2SO4. The model can be used as a preliminary tool to assess the rejection capability as well as the flux behaviour of NF membranes in mixtures of solutions. For future works, the model can be improved further to account for non-ideal as well as real mixture solutions.