Amorphous carbon nitride coatings (a-CNx) were deposited on SiC disk by ion beam assisted deposition (IBAD). The tribological behavior of aCNx coating sliding against SiC ball in water was investigated and compared with that of SiC /SiC system at room temperature. The influences of testing conditions on friction coefficient and specific wear rate of both kinds of tribo-pairs were studied. The worn surfaces on disks were observed by scanning electron microscope (SEM). The results indicate that the running-in period of a-CNx /SiC was shorter than that of SiC/SiC system in water. At a sliding velocity of 120mm/s, the mean steady state friction coefficients of SiC/SiC (0.096) was higher than that of a-CNx /SiC (0.05), while at 160mm/s, lower friction coefficient (0.01) was obtained for SiC/SiC in water. With an increment of normal load, the mean steady-state friction coefficients after running-in first decreased, reaching a minimum value, and then increased. For self-mated SiC, the specific wear rate of SiC ball was a little higher than that of SiC disk, while for a-CNx/SiC, the specific wear rate of SiC ball were 10 times smaller than that of a-CN x coating. Furthermore, the specific wear rate of SiC ball sliding against a-CNx coating was reduced by a factor up to 100-1000 in comparison to that against SiC in water. The wear mechanism of SiC/SiC system in water is related to micro-fracture of ceramic and instability of tribochemical reaction layer. Conversely, wear mechanism for a-CNx/SiC is related to formation and transfer of easy-shear friction layer.