This paper describes effects of contact pressure, sliding speed and ambient gas on the friction properties of a-CNx coating sliding against Si_3N_4. Friction coefficient decreases with the increasing of the contact pressure, and reaches to very small value (μ=0.01∿0.05). Sliding speed affects the contact pressure dependency of fiction coefficient. Friction coefficient increases with the humidity. The best ambient gas for very low friction coefficient is N_2. And the most effective method to introduce N_2 (μ=0.003).