A theoretical model of radiation heat transfer has been developed. A computation of radiation heat flux at a particular location in the combustion chamber by using the present model requires in-cylinder time- and space-resolved species data and cylinder pressure. From the species data, the burned fuel/air ratio distribution is inferred to compute space-resolved adiabatic flame temperature. For the computation of the spectral emissivity of an isothermal volume of adiabatic temperature containing soot, the Rayleigh-limit expression is used. The refraction indices in the expression are obtained by using the dispersion equations based on the electronic theory encompassing both free and bound electrons. For the spectral emissivity from the gaseous component in the volume, the semi-empirical band model is used. A parametric analysis of radiation heat transfer in diesel combustion is made by using the present model; a prediction by the model qualitatively compares with some of the reported experimental data.