In this paper, the effects of high concentrations of CO2 on the lower flammability limits (LFL) of the CH4/O2/CO2 mixture were studied. For comparison, the LFL of the CH4/O2/N2 mixture were studied in the same way. First, the LFL of gas mixtures were measured using a cylindrical quartz glass tube in the condition of various oxygen concentrations. The experimental values of LFL of CH4/O2/CO2 decreased with the increase of oxygen concentrations, but the decreasing rate was small. Then, the chemical, thermal, and radiative effects of high concentrations of CO2 on the LFL were analyzed with the energy balance analysis. The thermal property of the gas mixture played the major role in the determination of the LFL. The radiative effect of CO2 on the LFL was much smaller than the thermal effect. The chemical effect of CO2 has little impact on the LFL of CH4/O2/CO2. Finally, the LFL of the CH4/O2/CO2 mixture were well-predicted using the calculated flame temperature method with a fixed critical temperature.