The reaction mechanism of the Ethynyl radical and CH3CN has been studied by the Density Functional Theory (DFT) using the BH and HLYP functional in conjunction with the 6-311++G(d,p and, 6-311++G(3df,2p) basis sets. The potential energy surface (PES) for the C2H + CH3CN system was also established. Calculated results indicate that products of this reaction can be (HCCCH3 + CN), (C2H2 + H2CCN), (HCCCN + CH3), (H2CCNCCH + H), (HCCNC + CH3), (HC(CH3)C + CN) and (H2CC+CH2CN). However, the formation of (C2H2 + H2CCN), (HCCCN + CH3) and (HCCNC + CH3) is the most favorable. This study is a contribution to the understanding of the reaction mechanisms of the ethynyl radical with many small radicals and molecules in the atmosphere and combustion chemistry.