The carbon nitride (CNx) coating is one of the promising materials such as high wear proof, and superlow friction. It was reported that CNx coating showed superlow friction when it slid against a Si3N4 ball in N2 environment. From the view point of industrial field, 1.0 μm thickness and 20 GPa hardness is necessary to CNx coating. We investigated the effect of substrate temperature, nitrogen ion beam acceleration voltage, deposition rate, and the thickness on its hardness. The CNx coating was synthesized on Si(100) substrate with 5 min of nitrogen ion beam sputtering before synthesis. The coating paramaters were -20, -5, and 15 °C of substrate cooling and without cooling, 0, 0.3, 0.5 and 1.0 kV of nitrogen ion beam acceralation voltage, 0.7 to 2.1 nm/s of deposition rate, and 500 and 1000 nm of thickness. After the synthesis, the coatings were analized by Auger Electron Spectroscopy (AES), Raman, and indentation hardness. As a result, N ion beam accelerate voltage, deposition rate, and thickness was not effective to the hardness of CNx coating. On the other hand, it was clear that the CNx coating hardness increased from 10 to 20 GPa with decreasing of substrate temperature. From the Raman analysis, ID/IG ratio and G peak position decreased with decreasing substrate temperature. These results indicated that sp3 structure which included in CNx coating increased, and then the hardness of itself increased.