The effects of natural convection on self-ignition of solid fuel heated by external radiation were studied numerically. The numerical model includes thermal heating and subsequent degradation reactions in the solid phase with heat and mass transport of evolved degradation products into an atmosphere containing oxygen. The combustible fuel component of the product gas is mixed with oxygen to produce premixed gas and the heating of the gas accelerates the oxidation in the gas phase leading to spontaneous ignition. The induced natural convection was found to play a key role in the heat and mass transport, and hence in the time-dependent process leading to ignition. In the numerical calculation the magnitude of gravity was changed continuously to study its effects on the ignition time and ignition point. Effects of oxygen concentration in the atmosphere were also studied.