Three types of Ar incorporated DLC films were deposited using a thermal electron excited plasma CVD apparatus at bias voltages of -0.5 to -3 kV. Their friction and wear properties were evaluated under water and air environments using a ball-on-plate type reciprocating friction tester. The anti-wear properties of Ar-DLC films in water were superior to that in air; the smallest wear rate in water was approximately 8x10 - 9 mm 3 /Nm. The reduction in the friction coefficient in water was not clear, compared to that in air. The lowest friction coefficient in water was approximately 0.1. The transferred materials were observed on the wear scar of the mating ball in every experiment, but the transferred materials in water were greater in quantity than those in air. Furthermore, there seemed to be a qualitative difference in transferred materials between water and air environments, based on micro-Raman analysis. The difference in such transfer phenomena seemed to affect both the friction coefficient and wear rates. These friction and wear behaviors suggest that our Ar-DLC films are good candidates for use in rubbing machine parts when in use with water hydraulic systems.