The output characteristics of a TEA CO2 laser for various gas mixtures and various pump levels are quantitatively calculated using time-varying electron excitation coefficients. The calculation results are in good agreement with those of experiments. According to the theory of partial population inversion suggested by Polanyi, the conditions for partial population of CO2 molecules are discussed. The single peak of the output pulse for gas mixtures involving N2 is explained with the partial population inversion.