Naflon/silicon oxide composite membranes were produced via the in situ acid-catalyzed sol-gel reaction of tetraethylorthosilicate (TEOS) in Nafion perfluorosulfonic acid films. The TEOS uptake content of a composite membrane was varied according to the sol-gel reaction time. The physicochemical properties of the composite membranes were investigated by Fr-IR. The water uptake contents and the thermal properties of the composite membrane were analyzed by DSC-TGA. The performance of a membrane-electrode assembly (MEA) using the composite membrane was evaluated by single cell test and methanol permeability was measured by gas chromatography (GC). The results show that the performance of a MEA using the composite membrane was higher than that of a commercial membrane at high temperature.