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The separation efficiency of monoÀ/divalent cations by a commercial nanofiltration membrane can be enhanced by increasing the surface positive charge. In this study, a novel surface grafting method was suggested, including activation with 2-chloro-1-methyliodopyridine (CMPI), and sequent graft polyethylene-imine (PEI) and 2,3-epoxypropyltrimethylammonium chloride (EPTAC). Less flux reduction (<30%) and higher separation factor of Mg 2+ /Na + (8.03-9.77) was achieved with the modified membrane under the grafting condition of 0.05 wt% CMPI, 2 wt% PEI, and 1 wt% EPTAC. The modified membranes had higher surface positive charge, hydrophilicity and cross-linking degree. During the 32-h filtration of a mixed salt solution, the modified membranes showed larger permeance (>11 L m À2 h À1 bar À1 ) and higher ion selectivity than the pristine membrane, with separation factors of Mg 2+ /Na + and Mg 2+ /Li + ranging 36.3-46.2 for the modified membranes. The modified membranes also have higher antifouling properties and are more stable at higher pH condition. The results demonstrate a facile and scalable surface modification method to improve the cation selectivity of commercial NF membranes, which shows great potential in the application of monoÀ/divalent ion separation.