As a key element in the RED process, the membrane influences the overall power output of the RED stack. Especially, the membrane resistance is one of important properties of the membrane. In this study, CEM(cation exchange membrane) and AEM(anion exchange membrane) consisting of cross-linked polymers in a porous polyolefin substrate were developed to simultaneously satisfy high performance and cost-down of the applied RED system. Physico-chemically stable and ultrathin ion exchange pore filling membranes (cation exchange membranes and anion exchange membranes) with high ion transfer coefficients have been developed for reverse electrodialysis as an energy conversion system. The polymer electrolytes in both cases of cation exchangeable and anion exchangeable ones consisting of the whole hydrocarbon materials were introduced into porous hydrocarbon substrates and crosslinked by radical polymerization in this work. The thickness of the prepared pore filling membranes was controlled between 16 and 23 micrometers to extremely lower membrane resistance and to increase ion transfer. All the properties of the prepared membranes were compared with commercial membranes obtained from Tokuyama, Fuji, and Fuma-Tech. Furthermore, RED cell performance and energy efficiency using the prepared pore filling membranes were also evaluated.