The flammability limits of CH4/CO2/O-2 were studied using theory calculations and experimental investigation. A calculation model was re-deduced based on thermal theory. In this model, the chemical effects of CO2 on flammability limits are concerned. Results show that the chemical effects of CO2 decrease the upper flammability limits (UFL) of mixtures, while have little effect on the lower flammability limits (LFL). Experimental measurements were performed using a cylinder reactor. The measurements of the LFL of CH4/O-2/CO2 increase from 5% to 8.25% with the increase of CO2 concentration, and the UFL of mixture CH4/O-2/CO2 decrease from 61% to 8.25% accordingly. The model predicted the LFL very well, but the calculated values are an average 10.7% higher than the measurements at the UFL points. The differences between the predictions and measurements are mainly due to the two assumptions of this model. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.