This article illustrates a method to predict flammability of mixtures containing methane, oxygen, and other inert gases like nitrogen and carbon dioxide based on chemical equilibrium. Calculated adiabatic flame temperature (CAFT) is used as a parameter to determine whether a fuel mixture is combustible or not. Our calculated results are obtained with initial conditions of 1 atm and 298 K. Compared with experimental data reported in the literature, we find that 1,450 K is an appropriate threshold of flammability. Mixtures with adiabatic flame temperatures higher than this value are considered flammable while those lower are judged as nonflammable. Using this criterion, it is convenient for us to predict the flammable zone and prevent the existence of flammable mixtures to ensure security. Showing the CAFT of fuel mixtures with various concentrations in a triangular flammability diagram, we can easily find out the influence of inert gases, oxygen, and soot formation on the mixture's flammability, respectively. (c) 2014 American Institute of Chemical Engineers Process Saf Prog 34: 31-35, 2015