A method of computing longwave radiation transmissivities for water vapour and CO₂ has been proposed in this paper.According to the experimental data of absorption bands of water vapour and CO₂ we divide their spectra into 10 and 2 basic intervals, respectively, which we call basic wave bands. Some common formulas are derived and by the use of them we have calculated the transmissivities of water vapour and CO₂ at the temperatures 220°,260° and 300°K.The comparison of the results with those computed by the so-called accurate formulas and by Kuo's method shows that the method in this paper is good enough to obtain the transmissivities and better than Kuo's.