In3+　was　taken　as　a　new　dopant　to　codope　NaYF4　with　Er3+　and　Yb3+　by　hydrothermal　synthesis　in　this　manuscript.　How　to　improve　phase　transition　of　NaYF4　from　cubic　to　hexagonal　and　how　to　improve　upconversion　luminescence　performance　of　hexagonal　NaYF4　are　both　important　problems　because　hexagonal　NaYF4　is　the　most　efficient　matrix　material　for　upconversion.　Considering　these　two　problems,　we　report　the　study　of　crystal　structure　and　upconversion　emission　of　In3+-codoped　NaYF4　to　discover　phase　transition　mechanism　and　upconversion　luminescence　properties　changed　with　different　In3+　codoping　concentration.　NaY0.92Yb0.03Er0.02F4　was　tridoped　with　0,　2,　4,　6,　8　and　10　mol%　In3+　at　170　degrees　C　for　8　h　in　order　to　study　phase　transition　of　NaYF4.　The　other　group　of　samples　was　prepared　by　the　same　procedure　except　In3+　codoping　concentration　was　replaced　as　0,　1,　2,　3,　4　and　5　mol%　and　the　reaction　proceeded　at　190　degrees　C　for　12　h　in　order　to　obtain　pure　hexagonal　NaYF4　for　study　of　its　upconversion　luminescence.　The　change　of　morphologies　and　crystal　structural　transition　caused　by　In3+　codoping　was　explored　by　XRD,　Rietveld　refinement,　SEM　and　TEM　analysis　methods,　while　the　upconversion　luminescence　was　characterized　by　emission　spectra　and　luminescence　decay　curves,　for　the　relationship　between　crystal　microstructure　and　luminescence　properties　to　be　discussed　in　our　investigation.　On　the　basis　of　these　characterizations　we　found　that　with　rising　of　the　In3+　codoping　concentration,　the　phase　transition　of　NaYF4　from　cubic　to　hexagonal　was　improved.　The　intensity　of　upconversion　luminescence　was　also　found　to　be　increased　to　the　maximum　when　3　mol%　In3+　was　codoped　into　hexagonal　NaYF4.　The　analysis　and　discussion　discovered　that　the　phase　transition　of　NaYF4　was　correlated　with　the　lattice　distortion　of　cubic　NaYF4　improved　by　In3+　codoping.　The　intensity　of　upconversion　luminescence　was　proved　to　be　increased　by　the　asymmetry　of　the　local　crystal　field　changed　with　the　In3+　codoping　concentration.　This　investigation　may　be　helpful　for　design　and　synthesis　of　other　functional　materials.