This paper presents the results of the lower flammability limit (LFL) of C3H8/O-2/CO2 at normal temperature and pressure. The LFL of the C3H8/O-2/CO2 mixture was measured with an explosion vessel. The measured LFL of propane increased approximately linearly from 2.5% to 4.3% with the CO2 concentration rising from 5% to 70%. CO2 participates in element reactions, which decreases the combustion heat at LFL. The reduction of combustion heat is improved by the rising CO2 concentration and reached a maximum of 21.5% at 70% CO2 concentration. The effects of specific heat capacity and radiation property of the gas mixture on the LFL were analyzed. The effect of specific heat was improved by CO2, while the radiative effect was reduced.