In order to predict the characteristics of CO laser, usually nearly a hundred stiff. differential equations must be solved numerically. Thus it is of interest to make an analytical approach to this subject. Starting from basic governing equations for vibrational-distribution function and radiative intensities, we have deduced the gain-satura. tion relation of a steady CO laser, we consider the general oase in which both homogeneous and inhomogeneous broadening exist simultaneously for the radiative transiticn. The effects of multilevel cascade emission is elucidated. For two special cases of twolevel emission and homogeneous broadening limit, the present results are in agreement with the theories presented in references [9] and [4], respectively.