In　this　manuscript　we　report　a　phenomenon　that　upconversion　emission　intensity　of　Er3+　was　enhanced　while　decay　time　constant　was　decreased　obviously　by　Sn　codoping　with　Yb/Er　into　hexagonal　NaYF4　synchronously.　X-ray　powder　diffiraction,　field　emission　scanning　electron　microscope,　transmission　electron　microscopy,　X-ray　photoelectron　spectroscopy,　electron　spin-resonance　spectroscopy　and　upconversion　emission　spectra　were　employed　to　explore　the　relation　of　crystal　structure　and　properties.　From　these　characterizations　we　found　that　symmetry　of　the　rare　earth　ion　local　crystal　field　could　be　tuned　by　different　Sn　codoping　concentration.　For　the　variable　valence　property　of　Sn　the　local　crystal　field　asymmetry　and　emission　intensity　of　NaYF4:Yb,　Er　arrived　to　the　maximum　when　3　mol%　Sn　was　codoped,　while　decay　time　was　reduced.　The　study　of　this　changing　tends　of　upconversion　emission　intensity　and　decay　time　constant　may　be　helpful　for　design　and　fabrication　of　high　performance　upconversion　materials.　(C)　2013　Elsevier　Inc.　All　rights　reserved.