A microchannel electrochemical reactor system was designed to facilitate the reduction of CO2 to CO, which is composed of a premixing section in the upstream consisting of a circular microchannel for presaturation of CO2 in the electrolyte, and a reacting section in the downstream consisting of an annular microchannel containing an Ag rod for catalysis. Such a combined structure exhibited an ultra high 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 microchannel was increased by connection in series due to the prolonged contacting time between CO2 and the electrocatalyst.