A　series　of　NaLuF4-based　hexagonal　phase　upconversion　nanocrystals　(UCNs)　were　synthesized　by　a　facile　solvothermal　method　and　the　properties　of　the　UCNs　were　investigated.　The　results　show　that　the　as-prepared　nanocrystals　exhibit　pure　hexagonal　lattice　structures,　uniform　morphologies,　high　monodispersities　and　excellent　upconversion　luminescence.　The　upconversion　luminescence　(UCL)　intensities　of　the　UCNs　can　be　enhanced　by　coating　with　a　shell　of　NaLuF4.　More　interestingly,　the　UCL　intensities　of　active-shell　coated　nanocrystals　(NaLuF4:Gd,　Yb,　Er@NaLuF4:Yb,　Ho　and　NaLuF4:Gd,　Yb,　Er@NaLuF4:Yb)　are　remarkably　higher　than　that　of　inert-shell　coated　nanocrystals　(NaLuF4:Gd,　Yb,　Er@NaLuF4),　and　NaLuF4:Gd,　Yb,　Er@NaLuF4:Yb,　Ho　is　higher　than　NaLuF4:Gd,　Yb,　Er@NaLuF4:Yb.　The　mechanisms　of　upconversion　luminescence　enhancement　are　discussed　in　detail.　The　bioimaging　application　of　the　nanocrystals　showed　that　bright　upconversion　luminescence　was　observed　when　UCNs-labeled　HeLa　cells　were　excited　with　980　nm　light.　This　study　presents　a　facile　method　for　the　synthesis　of　NaLuF4-based　upconversion　nanocrystals　with　intense　luminescence　that　can　be　used　as　potential　fluorescent　probes　for　sensitive　bioimaging,　and　the　suggested　mechanism　could　provide　new　insights　into　fabrication　of　upconversion　materials　with　high　upconversion　fluorescence.