Sulfonated polystyrene-block-poly(ethyl-ran-propylene)-block-polystyrene (S-polySEPS) was prepared by sulfonation at the phenyl groups of the polystyrene-block-poly(ethyl-ran-propylene)-block-polystyrene (polySEPS) containing 65% styrene groups for proton exchange membrane. High proton-conducting S-polySEPS/silica nanocomposite films were produced by direct-mixing of nanosilica particles with the S-polySEPS copolymer. The TEM image of the S-polySEPS/silica nanocomposite films showed that the silica particles were very-well dispersed within the S-polySEPS matrix. Also, the XRD patterns showed the presence of the nano-scaled silica particles. Moreover, the nano-scaled silica particles played an important role in the prepared organic/inorganic nanocomposite properties such as proton conductivity, thermal stability, water content and ion exchange capacity (IEC). The S-polySEPS/silica 1 wt% (1.41 x 10(-1) S/cm) and 2 wt% (9.9 x 10(-2) S/cm) nanocomposite films had higher proton conductivity than Nafion 117 (9.8 x 10(-2) S/cm) at the temperature of 90 degrees C. The FT-IR analysis was used to verify the sulfonation of the S-polySEPS copolymer. The TGA analysis was carried out to investigate the thermal stability of the S-polySEPS/silica nanocomposite films.