The friction behavior and the mechanism of DLC films in CO₂ atmosphere are rarely explored, which is a significant obstacle for the potential practical application of DLC films in primarily CO₂ environment. Here, the experiments and first-principles calculations are performed to simultaneously investigate this theme. We find that DLC films in CO₂ atmosphere exhibit astoundingly low friction coefficient compared with in ambient air and vacuum atmospheres. The XPS and Raman spectrums demonstrate the possibly activation of CO₂ molecule in the shearing interfaces, which may be critical for the low friction of DLC films in CO₂ atmosphere. The calculated results reveal that the lactone groups can easily form during the horizontally chemisorption of CO₂ molecule on the DLC surface, which is energetic and is a favorable process under the interfacial stress. Because of the presence of the lone-pairs of the lactone group, the lactone-terminated surfaces appear to be responsible for the low friction of DLC films in CO₂ atmosphere. The studies may open up the possibility for DLC films usage in Mars applications.