Ammonia　is　regarded　as　one　of　the　potential　hydrogen　carriers　and　a　kind　of　carbon-free　renewable　energy　source.　However,　there　are　still　some　challenges　on　ammonia　application　in　combustion　devices,　i.e.,　the　high　NOX　emission.　In　this　study,　the　combustion　performances　and　emission　characteristics　of　co-firing　CH4/NH3/air　flame　with　NH3　mole　fraction　(ηNH3)　from　0　to　1.0　are　investigated　in　a　swirl　combustor.　The　emissions　in　the　exhaust　gas　are　analyzed　by　the　Gasmet　DX4000　Fourier　Transform　Infrared　(FTIR)　gas　analyzer.　The　simulations　of　the　chemical　reactor　networks　(CRN)　with　a　detailed　mechanism　are　employed　to　extend　the　understanding　of　experimental　data.　Results　show　that　the　combustion　efficiency　is　very　low　at　fuel　lean　conditions　for　high　ηNH3　flames,　and　NOX　emission　significantly　increases　when　blending　a　small　amount　of　ammonia.　The　co-firing　flames　produce　large　amount　of　NOx　at　lean　conditions　(　less　than　1.0).　There　exits　an　optimized　condition　(≈1.1)　where　the　NOx　and　NH3　emissions　reach　their　lowest　value　simultaneously.　The　co-firing　flame　produces　maximum　NOx　emission　at　ηNH3　=　0.5.　Therefore,　to　control　the　NOX　emission,　the　CH4/NH3/air　co-firing　flames　should　be　operated　far　away　from　ηNH3　=　0.5　at　rich　conditions.　NOX　reaction　pathway　analysis　shows　the　HNO　pathway　is　dominant　in　fuel　lean　conditions.　Thermal-NO　pathway　and　NHi　pathway　are　primary　in　stoichiometric　ratio　and　fuel　rich　conditions,　respectively.　©　2021　Elsevier　Ltd