A microchannel electrochemical reactor system was designed to facilitate the reduction of CO2 to CO2 which is composed of a premixing section in the upstream consisting of a circular microchannel for the presaturation of CO2 in the electrolyte and a reacting section in the downstream consisting of an annular microchannel containing a Ag rod as the catalyst. Such a combined structure exhibited an ultrahigh CO faradaic efficiency of 95.3% due to the efficient mass transfer under Taylor flow. It is interesting to find that CO2 conversion was increased from 0.94 to 3.66% when the length of the microchannel was increased by connection in series due to the prolonged contacting time between CO2 and the electrocatalyst. Furthermore, "scale-up" experiments demonstrated that the faradaic efficiency for CO could keep all above 95.0% at a current density of 7.5 mA cm(2) when the microchannel number was increased in parallel, revealing that the microchannel electrochemical reactor is efficient to conduct the electroreduction of CO2.