A large number of studies showed that nanoparticles used even in small quantities improve thermal stability and flammability resistance by reducing significantly the mass loss and heat release rate. Recently, a graphite nano-crystal and nano-clay have been used as an alternative to traditional fire retardants to improve the thermal stability and flammability resistance of polymeric materials. The objectives of this work are the experimental and theoretical evaluation of the pyrolysis and flammability of the polymer nanocomposites based on novolac type phenolic resin and graphite nano-crystal under the external convection and radiation heat flux. The theoretical results of heat release rate modeling based on conservation equations of mass and energy are then confirmed by the experimental data of thermal analysis. Nano-graphite shows good potential as flame retardant because upon pyrolysis of polymer, the nano-crystal graphite reinforced phenolic resin can be converted into a uniform carbon layer, which may lead to significantly higher resistance to oxidation compared to neat phenolic resin. Formation of this carbon layer on char formed from the pyrolysis of the nanocomposite at high temperature not only enhances the char mechanical strength, but also acts as secondary protection layer to protect the lower remained nanocomposite. •We examine the thermal and flammability analysis of novolac/graphite nanocomposites at different testes.•We analyse different alternatives for determination of the thermal performance of nanocomposites.•Thermal degradation behavior of nanocomposites were modeled and explained.