In this paper, the steady state behaviour, linear stability and the response to small signal modulations of cavity length, pump and gain or loss of a sigle mode CO2 laser with homogeneously broadening medieum are analysed based on the vibro-rotational model. The results are comparaed with the similar analysis for two-level rate-equation model. We put special emphasis on the effect of the non-lising Vibro-rotation levels on the response of the three modulations mentioned above.