This study investigated how the surface roughness of steel disks coated with a Si-containing diamond-like carbon (DLC-Si) film affects the dependence of the friction characteristics, in an oil-lubricated condition via the use of friction experiments and numerical analyses using mixed lubrication models. This study clearly demonstrated that a DLC-Si film with a surface roughness of RzJIS = 3 to 4 μm was an effective means of suppressing frictional vibration caused by stick-slip. In addition to finding that the surface roughness of steel disks coated with DLC-Si film could be used as a design guideline for the sliding surface of an electromagnetic clutch to provide excellent μ-v characteristics, it was also found that providing the mating plate with micro grooves that have a depth of 10 μm and a pitch of about 100 μm was an effective means of improving the friction characteristics. The results of this study suggested that the surface roughness and the micro grooves maintain solid contact even if the formation of oil film is enhanced as the sliding velocity increases, since these results in boundary lubrication.