The kinetics of gases (CO2, N2 and He) in an optimized se aled-off CO2 laser are discussed in details. A comparison of numerical results shows that the po pulation of the laser levels is increased by optimizing the lasing gases, with t he increase of an excited N2 impacting to the upper laser level. Also , decreas ing the electron momentum transition frequency and increasing discharge current result in the increase of the relaxation of the upper rotational laser levels an d the decrease of the level lifetime, thus the lasing saturation intensity in a resonator is increased finally.