We have investigated the effects of seven sputtering control factors on the hardness of carbon nitride (CNx) thin films by design of experiments and the analysis of variance (ANOVA) to synthesize hard CNx thin films. It was determined statistically that the substrate temperature, the sputtering pressure and the target to substrate distance are significant control factors for the hardness of the CNx thin films within the experimental range of this study. Especially, the distance is the most important control factor of the seven factors; the hardest films are obtained at the distance of 4.5 cm. On the other hand, the effects of the substrate treatment, the dc power, the nitrogen concentration and the sputtering time are not statistically significant. It is suggested that these statistical methods are effective to compare the importance of many sputtering control factors. The CNx thin films deposited under the optimized sputtering conditions exhibit a relatively high hardness value of 55 +/- 11 GPa, Young's modulus of 228 +/- 21 GPa and an elastic recovery (%R) of 98%. The compressive stress in the films is a low value of 0.3 GPa.