Abstract: The transmission of radiant heat in the tubular furnace has been studied. A new method of calculation is developed and a mathematical model is formulated. By means of this mathematical model, the heat of transmission for modern cracking tubular furnaces, heater of box type and cylindric heater can be solved more correctly. The new mathematical model for cracking tubular furnace is as follows.The mathematical model is derived from a new concept i. e. the flame-wall emissivity gw. Other types of mathematical models already in use are also discussed. As an example for illustration, six modern cracking tubular furnaces are calculated with the new method. The calculated and actual values are in agreement quite well. It is shown by these results that the proposed new model may be used in practical engineering calculations.