Using the sbsorption coefficients and the weighting factors of the gray gas, the total emissivities of C $O_{2}$- $H_{2}$O and C $O_{2}$- $H_{2}$O- transient species-soot gas mixtures can be expressed by the following equation, [a numerical formula] Where, $a_{i}$ and $K_{i}$ represent the weighting factor and the absorption coeffient of i-gray gas respectively; L is the pathlength of the gas. This equation is widely used for the analyses of the radiation heat transfer in the combustors of internal combustion engines and in the furnace of external combustion engines. In this work, a simple calculation model of the weighting factor and the absorption coeffient of the above equation was developed. The weighting factors and the absorption coefficients of combustion products were calculated by applying the model to various kinds of fossil fuels such as coal and heavy oil. Then, the computed total emissivities for each fuel and pathlength were compared with measured and calculated values which have been already published in the literatures. The followings were the results obtained through the comparisons between the calculated emissivites and the published values; the developed model for the calculations of the weighting factor and the absorption coefficient of C $O_{2}$- $H_{2}$O and C $O_{2}$- $H_{2}$O- transient species-soot gas mixtures could be applied over the wide ranges of the temperature and the pathlength; the errors between the total emissivities calculted and the values published were maximum 10%, and average 1%, respectivelylyly