Abstract: Hydrogenated amorphous carbon（a-C： H）films are widely used in industry owing to their unique combi‐ nation of properties such as high hardness， low friction， low wear and excellent chemical inertness. However， a-C： H films also show some inherent defects， which includes high internal stress， poor thermal stability， high coefficient of friction （0.1～0.4）in atmospheric environment. In this study， Si and O co-doped a-C： H films with low friction coefficients were deposited by plasma-assisted chemical vapor deposition（PACVD）technology using acetylene（C₂H₂）and hexamethyldisiloxane（HMDSO）as precursors. The microstructure and mechanical properties of Si and O co-doped a-C： H film were characterized by SEM， Raman， nano-indentation and scratch test. The results show that the Si and O co-doped a-C： H ex‐ hibits a typical structure of amorphous carbon， high hardness and elastic modulus， and good adhesion between film and substrate. The friction coefficients of the Si and O co-doped a-C： H films vary in the range of 0.04~0.08 in the atmospheric environment， and it is as low as 0.02 when the pressure of the atmospheric environment is 10 kPa and 5 kPa， indicating that the low friction coefficient of Si and O co-doped a-C： H film is associated to the combined effect of co-doping of Si and O dopants and the pressure of the service environment.