A numerical and theoretical investigation has been conducted on the flammability limits of n-heptane/air mixtures by considering the role of the cool flame chemistry. It is found that the existence of the cool flame can substantially extend the lean and rich limits of the conventional high-temperature flame. Different levels of kinetic criteria based on the controlling branching and termination reactions at low temperatures are derived and shown to agree well with the numerical results at normal and elevated pressures. Furthermore, the critical temperature of cool flame flammability limits at normal pressure is found to be around 600 K and is insensitive to pressure variations. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.