Commercial cation exchange membrane was modified using poly(1-alkyl-4-vinylpyridinium iodide) as polyelectrolyte. And the electrochemical properties and permselectivity between Ca²⁺ and Na⁺ of the modified membrane were investigated. Ion exchange capacity, water content and specific conductivity of the modified membrane was lower than the original membrane(CL-25T). The smaller the molecular weight of polyelectrolyte was, the larger the adsorbed amounts were for same dipping time. As the adsorbed amounts were larger, permselectivity was increased, but specific conductance and ion flux were decreased. Permselectivity was also increased with the formular weight of alkyl group, and specific resistance of Na⁺, Ca²⁺ was proportional to permselectivity.