Using　pulsed　laser　deposition　(PLD)　method,　we　have　prepared　nanostructured　GaN　films　of　the　same　thickness　on　Si　and　SiC　substrates,　and　analyzed　their　microstructure　characterization　and　field　emission　properties.　The　results　showed　that　the　substrates　of　GaN　nanostructured　films　had　significant　effect　on　the　microstructure　and　field　emission　properties.　Compared　with　the　GaN　nano-film　on　Si　substrate,　the　field　emission　from　the　GaN　nano-film　on　SiC　substrate　has　been　significantly　improved:　its　field　emission　current　was　increased　by　orders　of　magnitude.　The　field　emission　enhancement　should　be　originated　from　the　nanocrystalline　microstructure　and　its　orientation　polarization　induced　field　enhancement　effect.　Results　indicate　that　to　prepare　field　emission　films　of　outstanding　performance,　appropriate　substrates　and　crystal　microstructures　of　the　films　are　the　key　issues.