This study shows a new friction reduction method for a diamond-like carbon (DLC) coating using photocatalyst in water lubricated condition. It is well known that one of DLC coatings (e.g. hydrogenated amorphous carbon, a-C:H) exhibits low friction due to repulsive force generated by hydrogens on their top most surfaces. Thus, the low friction is mainly occurred in inert gas environment. On the other hand, if we can produce hydrogenated surface positively, low friction may be achieved in lubricated conditions. Here, it is well known that water electrolysis by photocatalyst generates hydrogen and oxygen atoms/molecules. In this research, we propose a new hydrogen terminating method for DLC surfaces with hydrogens generated by water electrolysis. Moreover, we clarified the effect of the hydrogen-end on lowering friction. We conducted friction tests using an a-C:H coating against a TiO2 disk (photocatalyst) in water lubricated condition. The friction test result showed a decrease in the friction coefficient which was associated with UV irradiation. It is considered that the reduction in hydrogen-bonding component of surface energy indicates a highly hydrogen-ended surface creation, resulting in higher repulsive force and lower friction.