Product selectivity control based on a liquid–liquid parallel laminar flow has been successfully demonstrated by using a microreactor. Our electrochemical microreactor system enables regioselective cross-coupling reaction of aldehyde with allylic chloridevia chemoselective cathodic reduction of substrate by the combined use of suitable flow mode and corresponding cathode material. The formation of liquid–liquid parallel laminar flow in the microreactor was supported by the estimation of benzaldehyde diffusion coefficient and computational fluid dynamics simulation. The diffusion coefficient for benzaldehyde in Bu₄NClO₄-HMPA medium was determined to be 1.32 × 10⁻⁷ cm²s⁻¹ by electrochemical measurements, and the flow simulation using this value revealed the formation of clear concentration gradient of benzaldehyde in the microreactor channel over a specific channel length. In addition, the necessity of the liquid–liquid parallel laminar flow was confirmed by flow mode experiments.