In this study, the effect of CO2 reactivity on low NOx 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 NOx burner. The primary relative O2/CH4 ratio (λprimary) was 0.6 or 0.7, and the total relative O2/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 CO2 + H → CO + OH. N2 formation provided useful information to discuss low NOx combustion because an increase in the N2 yield indicated low NOx combustion. At λprimary = 0.7, the N2 yield decreased with increasing inlet CO2 mole fraction. Meanwhile, the N2 yield increased with increasing inlet CO2 mole fraction at λprimary = 0.6, regardless of the gas temperature. Sensitivity analysis showed that rate-limiting reactions for N2 formation were changed as λprimary varied. In fact, NH increased the rate-limiting reaction for N2 production at λprimary = 0.7, while not only NH but NH2 increased the rate-limiting reactions for N2 production at λprimary = 0.6. At λprimary = 0.7, most of the NH3 and HCN were decomposed; however, at λprimary = 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 N2 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 N2 formation.