To develop a high performance direct methanol fuel cell, a novel electrolyte membrane is needed. This electrolyte membrane should be durable up to 130degreesC to improve the catalytic reaction, and the methanol crossover should be reduced. Our approach was to design a pore-filling-type polyelectrolyte membrane, where the polyelectrolyte is filled into the pores of a porous substrate. This makes an integrated system with a membrane and a catalyst layer. The porous substrate was completely inert to aqueous methanol solution and was durable at high temperature. The substrate matrix could suppress membrane swelling to reduce methanol crossover, and showed mechanical strength at high temperatures. A radical polymerization technique was employed to fabricate the pore-filling membrane. A porous silica sol-gel thin base membrane on a carbon electrode was used as a membrane-electrode integrated system. The substrate pores were filled with a poly(acrylic acid-co-vinyl sulfonic acid) network. The membranes showed high proton conductivity, thermal stability, and low methanol permeation.