Only relatively hard materials, such as ceramics and steel, have been often studied as counterpart materials in testing the tribological properties of DLC films. Therefore the tribological properties of DLC films sliding against both hard and soft mating materials were studied in this paper. The transfer of the amorphous carbon to the slider is not the only reason for the low friction coefficient of DLC films. The friction properties of DLC films are influenced by slider hardness. In sliding against hard materials, DLC films exhibit a low, stable, friction coefficient. In sliding with easy-transfer soft metals, their friction coefficient is also low: In sliding with the difficult-transfer soft metals, DLC films exhibit a high friction coefficient. The wear rates of DLC films are strongly related to the mating material hardness: the harder the slider, the higher the wear rates. With easy-transfer soft metals, the formation of soft metal transfer films on DLC caused the low friction behavior. DLC films undergo phase transformation during sliding, changing from a diamond-like structure to amorphous carbon rather than to graphite.