We investigated diamond-like carbon (DLC) coatings with different thicknesses deposited on orthodontic stainless steel (disks and wires) using plasma-based ion implantation/deposition method. The frictional force of the disk or wire/bracket combination specimens was measured. The coating layers were characterized by scanning electron microscopy, atomic force microscopy, contact angle measurements, X-ray diffraction, nanoindentation analysis, and a three-point bending test. The adhesion of Streptococcus mutans Ingbritt on the disk specimen was evaluated. The thin DLC coating reduced the frictional force due to its hard surface. However, relatively thick DLC layers ruptured from the DLC–steel interface during the drawing friction test; additionally, the thicker layers adversely affected the frictional force. DLC layer thickness influenced the three-point bending properties. The DLC layer with higher wettability exhibited significantly lower adhesion potential for S. mutans than the non-coated specimen. Thus, surfaces modified with the addition of DLC coating demonstrated improved frictional properties and bacterial adhesion.