Europium-doped　gallium　nitride　(Eu:GaN)　nanocrystals　with　hexagonal　wurtzite　structures　were　prepared　from　the　decomposition　of　the　EuxGa1-x(H2NCONH2)6Cl3　precursor　at　a　mild　temperature　of　350　°C.　The　absorption　spectrum,　the　excitation　spectra　and　the　fluorescence　emission　spectra　of　Eu:GaN　nanocrystals　are　measured　and　discussed.　Eu:GaN　has　a　strong　fluorescence　emission　band　in　the　visible　region.　Furthermore,　the　first-principles　density　functional　theory　has　been　adopted　to　explore　the　fluorescence　mechanism.　The　results　show　that　energy　transfer　should　occur　between　the　GaN　host　and　Eu3+　ions　under　excitation　at　370　nm,　and　thus　the　fluorescence　emission　efficiency　of　the　Eu3+　ions　can　be　improved.　In　order　to　further　reduce　the　loss　of　fluorescence　emission　and　increase　the　fluorescence　quantum　yield,　Eu:GaN　nanocrystals　coated　by　biocompatible　silica　and　dispersed　in　ethanol　are　also　discussed.　©　2017　The　Royal　Society　of　Chemistry.