Abstract: The amorphous hydrogenated carbon (a-C:H) film was deposited by medium frequency unbalanced magnetron sputtering. The tribological properties of the film in high vacuum (5.0×10^-3 Pa) under different applied loads were evaluated by using a pin-on-disk tribometer. The influence of stress (contact stress and internal stress) on vacuum tribological properties of the film was investigated by observing the worn surface and the verified experiments. The results show that both the friction coefficient and sliding lifetime of the film decreased with increasing applied load. The surface of the film showed significant stress relaxation pattern under either friction or static pressure condition in high vacuum with high contact stress. Therefore, it can be regarded as that the extremely short sliding lifetime of the film in vacuum was closely related with the relaxation of internal stress under high contact stress.