The　nanocrystalline　titanium　dioxide　with　an　average　particle　diameter　of　4.　0nm　was　synthesized　in　Nafion　membranes.　The　crystals　of　titanium　dioxide　are　dispersed　singly　in　Nafion　membrane.　High　resolution　transmission　electron　microscopy(HR　TEM),　X-ray　diffraction　(XRD)　and　Uv-vis-nir　spectrophotometer　were　attempted　to　characterize　the　morphologies,　grain　size　distribution,　microstructure　of　Nafion　membranes　and　formation　mechanism　of　the　particles.　The　results　show　that　the　quantity　of　Ti4+　in　Nafion　depends　on　the　concentration　of　Ti　(OC3H7)(4)　in　soaking　solution　beside　the　kind　of　precursor.　The　formation　rate　to　transform　titanium　isopropoxide　to　TiO2　is　controlled　by　the　diffuse　of　water　to　enter　Nafion　via　the　channels.　The　size　of　nanocrystalline　matches　with　the　diameter　of　cavities　in　Nafion,　and　is　independent　of　the　kind　of　precursor,　the　concentration　of　Ti4+　and　Crystallization　condition.　The　effects　of　changing　the　variables　of　the　preparation　condition　on　the　formation　of　nanocrystalline　titanium　dioxide　are　discussed　in　detail.