Frictional behaviour of an unhydrogenated diamond-like carbon (DLC) coating deposited using the combined closed field unbalanced magnetron sputter ion plating and plasma assisted chemical vapour deposition (CFUBMS-PACVD) technique has been investigated in vacuum, in air with various relative humidity (RH) levels as well as in water. At low RH levels and in vacuum, a running-in period was observed where the initial friction coefficient was high and fluctuating and gradually decreased to reach some steady state value. Above approximately 40% RH, low friction coefficient was observed immediately after the sliding was started. While there was very little difference in the steady state friction coefficient among the various RH levels, the initial friction coefficient at the beginning of sliding decreased significantly with increase in relative humidity. Mechanisms for the observed phenomena are discussed based on the structure of the DLC coating and the passivation of the DLC coating surfaces by water adsorption.