An experimental study of plasmachemical reaction involving CH4 and N-2 molecules in rf discharge was studied in order to know the effect of vibrational excitation of N-2 molecules. When the relative nitrogen concentration was greater than 0.8, the main product of CH4 decomposition was HCN, and the rate of methane decomposition at this condition was faster than that one in pure methane. These results could be confirmed through the mass spectroscopic method. The reason for these results is the vibrational energy of N-2 excited by rf discharge. The chain reaction mechanisms of producing HCN by vibrational excitation of N-2 were examined closely through numerical simulation. The rate-controlling step was the dissociation reaction of excited nitrogen molecule to the atomic nitrogen, so the process of HCN synthesis was limited by the value of reaction constant, k(N).