||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 (ϕ less than 1.0). There exits an optimized condition (ϕ≈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.