Abstract: The hydrogen-free diamond-like carbon (DLC) films can produce low coefficient of friction in humid atmosphere, which mainly due to the passivation of " dangling bonds” on the surface of carbon-based thin films by H₂O and O₂ active molecules. However, the mechanism of the synergistic effect of the two molecules on the low-friction behavior of hydrogen-free DLC remains is unclear. The first-principles calculations were performed to investigate the passivation state of diamond surface when H₂O and O₂ molecules coexisted, and the possible ways of achieving low friction with hydrogen-free DLC films were inferred. The results show that H₂O and O₂ molecules were decomposed to form OH, H and O. At the same time, the mutual attraction of O atom and H atom promoted the formation of more OH groups. When H₂O and O₂ molecules coexisted in the ratio of 2:1, the diamond surface was completely passivated by the OH group. In contrast, the diamond surface formed a complicated situation in which C-OH, C-H, and C-O coexisted.