The influence of surface morphology of diamond films on their friction behaviors was investigated under both ambient air and water environments. The diamond films with three different surface morphologies were deposited on the Co-cemented tungsten carbide (WC-6wt%Co) substrate using hot filament chemical vapor deposition (HFCVD) method and their frictional behavior were studied on a ball-on-plate type reciprocating friction tester, where the ball-bearing steel was used as the counterface materials. The coefficient of friction (COF) of diamond films showed strong surface morphology dependence, those samples with smaller diamond grain size and smoother surface finish represented lower COF in both dry and water lubricating conditions. Water lubricating was significantly favor to decrease the COF of diamond films, which might mainly due to the lubricating effect of physisorbed water layer formed on the sliding interface. In addition, the transferred materials layer adhered on the worn surface of diamond films were supposed to be another factor that was beneficial for reducing the COF.