The　N+　ion　implantation　method　was　used　to　improve　the　field　emission　characteristics　of　polycrystalline　diamond　films.　The　emission　characteristics　after　ion　implantation　were　compared　with　those　before　implantation.　It　was　found　that　there　exist　considerable　differences　in　the　threshold　electric　field　and　emission　current　density　even　for　samples　fabricated　under　the　same　process　conditions　before　N+　ion　implantation.　These　differences　were　eliminated　by　N+　ion　implantation　treatment.　We　believe　these　differences　among　the　samples　are　due　to　the　difference　in　distribution　of　the　defect　energy　levels　in　the　forbidden　gap　in　the　diamond　films,　and　that　the　difference　in　distribution　of　the　defect　energy　levels　can　be　essentially　eliminated　by　ion　implantation.　Emission　current　density　under　high　field　also　increases　after　ion　implantation.　The　improvement　mechanism　was　analyzed　theoretically,　based　on　a　model　we　have　presented　before.　©　2002　Sage　Publications.