On the assumptions that a radiating gas mass having a uniform coefficient of absorption exists in the center of the furnace and that between radiating gas mass and walls exists transparent gas, the emissivity of a gas mass radiating to furnace walls and the frequency distribution of the heat-flow intensity on furnace walls are calculated. When the volume ratio of radiating gas to furnace is about 0.2, the frequency distribution calculated coinsides closely with that of actual furnaces, which is independent of the form of furnace or operating conditions. The average absorptivity of the gas mass for the radiation between any two of the furnace walls is also presented in the present paper. The results will be able to be utilized for calculating the heat absorption in any one of the furnace walls.