The　(002)　oriented　GaN　nanostructured　films　prepared　by　pulsed　laser　deposition　(PLD)　on　n-type　Si　(100)　substrates　were　treated　by　H-　and　O-plasma,　finding　both　treatments　do　not　make　the　sizable　changes　in　crystalline　structure　and　morphology　of　GaN　nanofilms.　However,　the　field　emission　(FE)　performance　of　GaN　nanofilms　is　considerably　improved　by　H-plasma　treatment　(H-GaN)　but　deteriorated　by　O-plasma　treatment　(O-GaN).　The　turn-on　fields,　Eon,　for　H-　and　O-plasma　treated　GaN　nanofilms　are　determined　to　be　0.52　and　1.79　V　μm−1,　respectively,　in　contrast　with　that　of　pristine　GaN　nanofilms,　0.95　V　μm−1.　The　improvement　of　FE　performance　by　the　H-plasma　treatments　could　be　attributed　to　the　reasons　of　the　reduced　surface　potential　barrier,　the　increased　electron　concentration,　the　increased　surface　conductivity,　and　the　increased　effective　emission　area.　The　first-principles　calculations　represents　that　the　experimental　and　theoretical　work　function　results　have　the　same　trend　with　the　descending　sequence　of　O-plasma　treated　GaN　＞　pristine　GaN　＞　the　H-plasma　treated　GaN.　©　2021　IOP　Publishing　Ltd