Friction behaviors of hydrogenated diamond-like carbon (DLC) film sliding against steel ball were investigated on a ball-on-disk test rig in different environment. The worn surface morphology of the steel ball was observed on a scanning electron microscope (SEM), and the chemical states of some typical elements on the worn surface of DLC film were investigated by means of X-ray photoelectron spectroscopy (XPS). The result showed that the friction behaviors of hydrogenated DLC film were very sensitive to the testing environment. In dry N-2, the film provided a much low and stable friction coefficient of about 0.035. When the friction tests were performed in higher relative humidity and in dry 02, higher and unstable friction coefficient of more than 0.2 was obtained. It was found that in oxygen and/or water containing environments, the transferred carbon-rich layer on the counterpart steel ball was inhibited, and the friction-induced oxidation of the DLC film surface and the chemical reactions between the DLC film and steel ball were involved in the friction processes, which accounted for the higher friction coefficient of the DLC film. The roles of environment in the friction behaviors of DLC film were discussed in terms of the friction-induced physical and chemical interactions among the DLC film, steel ball and water and/or oxygen molecules. (c) 2004 Elsevier B.V. All rights reserved.