Silver (Ag)/diamond-like carbon (DLC) nanocomposite films with different Ag concentrations ranging from 0 to 11.02 at.% were prepared by medium frequency unbalanced magnetron sputtering, in which the mixed Ar/CH4 of different volume ratios were used as the source gases. The doping effects of Ag concentration on microstructure, mechanical and vacuum tribological properties of the DLC films were investigated. It is found that the Ag concentration increased with the increasing Ar/CH4 ratios, accompanied with the increasing number and size of Ag crystalline. With moderate incorporation of Ag at 3.55 at.%, the film (deposited at the Ar/CH4 = 65/45) maintained a low internal stress without considerable decrease of hardness and thus improved the adhesion strength. Moreover, the film showed low friction coefficient and the longest sliding lifetime in vacuum. The significant improvement in tribological properties of Ag/DLC nanocomposite films with moderate Ag concentration can be attributed to the low shear strength of Ag clusters on the surface, as well as the diffusion of Ag from the bulk to the surface and wear track. (C) 2013 Elsevier B.V. All rights reserved.