The main mode of heat transfer of combustion gases at high temperature is thermal radiation of nonluminous gases, CO2 and H2O. Therefore the information of the emissivities of CO2 and H2O would be very important in the thermal performance analysis of furnace. In this study, an exponential model for the emissivities of CO2 and H2O was derived as function of P$_{g}$L and polynomial of reciprocal of temperature. Error analysis between the calculated values from present model and the valued of Hottel Chart was performed over temperature range of 1000-5000 R and a partial-pressure-length product range of 0.003 to 20 ft-atm. For CO2 gray gas, the error percent between the calculated values and the values from Hottel Chart was distributed within 2.5% in case of using a polynomial in 1/T of degree 4. For H2O gray gas, the model has an error range of 0 to 2.5% in case of using a polynomial in 1/T of degree 3.