The electrocatalytic activity of Cu electrode in the electrochemical reduction of carbon dioxide (CO2) was investigated. Electroreduction mechanism of CO2 was studied by the adsorption/desorption behaviors of reacting species by using an in-situ electrochemical quartz crystal microbalance. (EQCM) and the surface changes measured by ex-situ SEM, AES, and XRD analysis. During cathodic reduction of CO2 on Cu, the adsorption of amorphous carbon was observed. After electrolysis time of 1 h at constant cathodic potential, the poisoning of amorphous carbon resulted in the decrease of the faradaic efficiency for the formation of hydrocarbons such as CH4 and C2H4. On the other hand, the potential modulation method caused the change of the surface structure of copper, i.e. the formation of cuprous oxide (Cu2O). This structural change prevented the adsorption of amorphous graphite and the constant production rate of methane was obtained in long-term electrolysis.