Using　thioacetamide　(TAA)　as　precursor,　homogeneous　and　steady　monodispersed　CdS　nanocrystals　were　synthesised　in　Perfluorinated　Ionomer　Membranes　(Nafion)　by　self-assembly.　Unlike　the　ionic　precursors　such　as　Na　2S　and　H2S,　TAA　could　diffuse　into　the　cationic-exchangeable　ionomer　membranes　much　more　uniformly,　Which　led　to　the　homogeneous　distribution　of　CdS　nanoparticles　in　the　Nafion　membranes.　High　resolution　transmission　electron　microscopy(HRTEM),　X-ray　diffraction(XRD)　and　energy　dispersive　X-ray　analysis(EDXA)　were　used　to　investigate　the　synthesis　mechanism,　the　size　distribution　and　the　microstructure　of　these　CdS　nanoparticles　embedded　in　Nafion　membranes.　Optical　properties　of　CdS　nanoparticles　in　Nafion　were　also　characterized　by　UV-Vis　absorption　and　luminescence　spectra.　The　results　show　that　the　quantum　size　effect　becomes　strong　with　the　decrease　of　the　size　of　CdS　nanocrystals.　Blue　shift　appears　both　on　the　absorb　edges　in　the　absorption　spectra　and　on　the　band-edge　emission　in　the　luminescence　spectrum.