The tribological behaviors of a-CNx/SiC tribo-pairs in water at low velocity were investigated and compared with SiC/SiC at room temperature. The results showed that the mean value µ of friction coefficients at steady state for a-CNx/SiC tribo-pairs (µ=0.075~0.12) was lower than that of SiC/SiC systems (µ=0.24~0.27). At a constant sliding speed, µ for two kinds of tribo-couples decreased with an increase in normal load, but at a constant normal load, µ for SiC/SiC system was nearly independence of sliding speed, whereas that of a-CNx/SiC tribo-pair decreased with sliding speed. The specific wear rates (ws) of ball and disk for a-CNx/SiC tribo-couples were considerably reduced by a factor up to 10 in the comparison to those of SiC/SiC systems. The observations of wear scars showed that the wear mechanism of CNx coating was micro-fracture of irregularities on the film surface for build-up of tribolayer, while that of SiC/SiC system was dominated by mechanical wear.