The electrochemical carbon dioxide reduction reaction (CO 2 RR) to high value-added fuels or chemicals driven by the renewable energy is promising to alleviate global warming. However, the selective CO 2 reduction to C 2 products remains challenge. Cu-based catalyst with the specific Cu 0 and Cu + sites is important to generate C 2 products. This work used nitrogen (N) to tune amounts of Cu 0 and Cu + sites in Cu 2 O catalysts and improve C 2 -product conversion. The controllable Cu 0 /Cu + ratio of Cu 2 O catalyst from 0.16 to 15.19 was achieved by adjusting the N doping amount using NH 3 /Ar plasma treatment. The major theme of this work was clarifying a volcano curve of the ethylene Faraday efficiency as a function of the Cu 0 /Cu + ratio. The optimal Cu 0 /Cu + ratio was determined as 0.43 for selective electroreduction CO 2 to ethylene. X-ray spectroscopy and density functional theory (DFT) calculations were employed to elucidate that the strong interaction between N and Cu increased the binding energy of NCu bond and stabilize Cu + , resulting in a 92.3% reduction in the potential energy change for *CO-*CO dimerization. This study is inspiring in designing high performance electrocatalysts for CO 2 conversion.