Abstract: Diamond like coatings (DLC) were deposited on silicon substrate using a filtered cathodic arc plasma deposition system. The morphology and hardness of the resulting DLC coatings were observed and determined with an atomic force microscope and a nano indenter, respectively. The friction behavior of the DLC coating sliding against SAE 52100 steel at various loads and relative humidity was investigated on a DF PM unidirectional friction coefficient measurement apparatus. The morphologies of the wear scar of the coating and of the counterpart steel ball were observed with a scanning electron microscope. The results showed that the DLC coating sliding against the steel at a lower relative humidity registered a lower friction coefficient with small fluctuation, while the friction coefficient rose significantly at increased load and relative humidity. This was attributed to the different formation mechanisms of graphitized transfer film on the counterpart steel surface. Since the graphitization and transfer film formation of the DLC coating on the counterpart steel was restrained in the presence of water, catastrophic destruction of the film occurred after only 400 sliding at a relative humidity of 100%. Thus the DLC coating prepared was not suitable to the protection of Si based elements working in environment of high relative humidity.