Sulfate removal from an aqueous system having ternary ions consisting of sodium and magnesium cations was modeled using Donnan-Steric-Pore model (DSPM). The ability of DSPM to characterize a membrane on the basis of diffusion, convection and electromigration (Donnan effect) makes it suitable for this process. A flat sheet hydrophilized polyamide (HPA50) nanofiltration membrane was used for experimental work. Membrane parameters such as hydraulic permeability (4.2*10-12 m3 m-2 s-1 Pa-1), effective pore radius (rp) and membrane charge density (Xm) were estimated. The effect of various operating parameters such as feed concentration, transmembrane pressure and feed flow rate was also studied for membrane performance. The model prediction was compared with the experimental data, and it was found that model prediction ability was excellent. The model was capable of predicting permeate concentration, and rejection percentage within 8% error was observed. This confirms the efficacy of our proposed model.