Nitrogen-doped sp2 nanocarbon materials have been considered promising catalysts for CO2 electro-reduction. However, a fundamental understanding about product selectivity and the reaction routes is still lacking. In this report, the reaction mechanism on nitrogen-doped sp2 nanocarbon materials is resolved by clarifying the authentic origin of the carbonaceous products: CO and HCO2−. Two carbon-reduction pathways are identified based on a series of comparative studies by using differential electrochemical mass spectrometry and in situ CO adsorption experiments: a dominant pathway leading to CO from CO2 and a secondary pathway leading to HCO2− from HCO3−. Neither hydrocarbon (CxHy) nor alcohol or aldehyde (CxHyOz) were detected in the reduction of CO2. However, CO, which is generally regarded as an intermediate to be transformed into these products on metal catalysts, can undoubtedly be produced and adsorbed on nitrogen-doped sp2 nanocarbon catalysts during the reaction.