In this study, an experimental apparatus was developed where the relative humidity (RH) and oxygen concentration could be controlled between 0.01-10%RH and 1-100000 ppm, respectively. This apparatus was used to study the roles of water and oxygen molecules on the friction of carbon nitride (CN_X) coatings. The friction properties of Si₃N₄ balls sliding against CN_X (Si₃N₄/CN_X) under a nitrogen atmosphere were classified into the following three modes according to their average friction coefficients (μ_a) and the stability of the friction: Mode I, μ_a < 0.05 (stable); Mode II, 0.05 < μ_a < 0.3 (stable); and Mode III, 0.3 < μ_a (unstable). To achieve a low friction coefficient (e. g. < 0.05), the optimum RH and oxygen concentration were 0.1-1.0%RH and 100-10000 ppm, respectively. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) showed that hydrogen and hydroxyl radicals derived from water molecules chemisorbed onto the worn surfaces of the CN_X coatings. Raman spectroscopy suggested that the structure of carbon on the worn surfaces was different from that of the deposited CN_X coating. We concluded that low friction (Mode I) arose from the water and oxygen molecules terminating carbon dangling bonds and structural changes in CN_X.