The objective of this research is to investigate the frictional behavior of a-C:H coating in plane contact by using reciprocating micro-sliding test under controlled relative humidity. The friction tests were conducted at various relative humidities (RH) between 5% and 90% without lubrication at room temperature under the following conditions: normal load of 1 N, sliding stroke of 2 μm, sliding speed of 10 μm/s, and up to 80000 cycles. The friction coefficient of the DLC coatings first increased and then decreased with increasing relative humidity when the number of sliding cycles was below 30000. Critical relative humidity was noticed, which corresponded to the highest coefficient of friction. At a relative humidity of 80%, the friction coefficient became almost constant over the whole number of sliding cycles.