We prepared structurally stable organic-inorganic hybrid ionomer membrane in which nano-sized solid proton conductors were uniformly dispersed in an ion exchange polymer matrix. Nafion membrane was in-situ doped with zirconium hydrogen phosphate (ZHP) after phase-separated hydrophobic porogen, dibutyphthalate, was leached out from Nafion membrane by diethyl ether and methanol co-solvent. FE-SEMicrographs showed that nanoporous structure in Nafion membrane was well developed by the solvent extraction process, and the pores were completely filled with in-situ doped ZHP particles. It was confirmed by FTIR study that hydrophilic ZHP fillers improved water retention of composite ionomer membrane at high temperature regions above 100degreesC Consequently, the high temperature conductivity of Nafion/ZHP membranes was much higher than that of neat Nafion membrane.