||Partial cation exchange membranes were prepared by direct sulfonation reaction of Poly(VDF-co-hexafluoropropylene)copolymers(PVDF-co-HFP). The structure of sulfonated PVDF-co-HFP was confirmed by FT-IR, 1H nmr and X-ray Photoelectron Spectroscopy (XPS). The membrane properties such as water uptake, ion exchange capacity, electrical resistance, and ion conductivity were measured. The maximum values of ion exchange capacity and ion conductivity were 0.89 meq/g and 0.0026 S/cm, respectively. The minimum values of water uptake and electrical resistance were 13.9% and 3.70 Ω • cm2, respectively. Therefore, it was identified that the optimum direct sulfonation temperature of PVDF-co-HFP ion exchange membranes are between 45℃ and 60℃ and below 7 hours. In the MCDI process, the TDS removal rate was measured. And the TDS removal rates were compared to the commercial membrane, CMX in a variety conditions.