In this study, the effect of CO2 reactivity on low NO combustion by varying CO2 mole fraction in inflow gas was experimentally and numerically investigated. A flat CH4 flame doped with NH3 for fuel N was formed in a reactor allowed secondary gas injection to simulate the fuel-rich region in a low NO burner. The primary relative O-2/CH4 ratio (lambda(primary)) was 0.6 or 0.7, and the total relative O-2/CH4 ratio was set to 0.8 by injecting a secondary gas. Measurement showed excited OH radical increased with increasing inlet CO2 mole fraction, and calculation showed that OH radical formation increased with increasing inlet CO2 mole fraction through the CO, + H -> CO + OH. N-2 formation provided useful information to discuss low NOx combustion because an increase in the N-2 yield indicated low NO combustion. lambda(primary) = 0.7, the N-2 yield decreased with increasing inlet CO2 mole fraction. Meanwhile, the N-2 yield increased with increasing inlet CO2 mole fraction at lambda(primary) = 0.6, regardless of the gas temperature. Sensitivity analysis showed that rate-limiting reactions for N-2 formation were changed as lambda(primary) varied. In fact, NH increased the rate-limiting reaction for N-2 production at lambda(primary) = 0.7, while not only NH but NH2 increased the rate-limiting reactions for N-2 production at lambda(primary) = 0.6. At lambda(primary) = 0.7, most of the NH3 and HCN were decomposed; however, at lambda(primary) = 0.6, some amounts of NH3, HCN, and CH4 remained. When minimal amounts of NH3 and HCN,remained, the dominant role of OH radicals was to oxidize NH, which was an important NO reducing agent. However, when greater amounts of NH3 and HCN remained, the OH radicals produced NH2 by oxidizing NH3. Moreover, sensitivity analysis showed that H radical formation plays an important role in N-2 formation under fuel-rich conditions. Hydrocarbon decomposition is needed to produce the H radical. The OH radical was active in CH4 decomposition, and CO2 could react with hydrocarbon radicals; thereby, with an increase in inlet CO2 mole fraction, exhaust CH4 mole fraction decreased, and the H radical increased. This resulted in the enhancement of N-2 formation.