The　formation　of　crystalline　titanium　dioxide　(TiO2)　nanoparticles　in　Nafion　membrane　structure　was　investigated.　The　X-ray　powder　diffraction　and　high-resolution　transmission　electron　microscopy　(TEM)　results　show　that　these　nanoparticles　are　pure　anatase　TiO2.　The　TEM　results　also　show　that　the　crystalline　TiO2　nanoparticles　are　ellipse　in　shape,　with　an　estimated　average　aspect　ratio　(long　axis/short　axis)　of　1.4.　The　average　size　of　the　TiO2　nanoparticles,　which　is　independent　of　the　loading　of　titanium,　is　similar　to　the　estimated　dimensions　of　the　hydrophilic　cavities　in　Nafion　membrane　in　terms　of　the　ion　cluster　model.　However,　there　is　no　evidence　for　any　TiO2　nanoparticles　in　the　proposed　channels　that　connect　the　hydrophilic　cavities.　The　fact　that　the　TiO2　nanoparticles　are　distributed　homogeneously　throughout　the　Nafion　membrane　structure　suggests　that　the　nanoparticles-embedded　membrane　still　maintains　a　certain　degree　of　porosity.　The　properties　and　potential　applications　of　the　Nafion-supported　crystalline　TiO2　nanoparticles　are　discussed.