From the view point of reformation of structure, we focused on ultraviolet light (UV) irradiation. It was assumed that UV was able to destruct the a-C:H topmost surface to graphite-like when the light intensity exceeded C-C single bond energy, and generated dangling bonds were assumed to combine. We prepared PECVD a-C:H coatings which were irradiated UV of 254, 312, and 365 nm wavelength at 60min. These specimens and as-deposited a-C:H was tested with ball-on-disk type friction tester, done with 0.1N, 0.08.4mm/s, and counter material was Si_3N_4 balls with 8 mm diameter at poly-alpha olefin lubrication. The a-C:H with 254 and 312 nm wavelength irradiation showed friction coefficient 0.047 which was lower than as-deposited a-C coating. The surface roughness of UV irradiated a-C:H did not change. These results indicated that structural change of the topmost surface to graphite-like structure was important roll to obtain low friction coefficient.