This article describes the highly efficient asymmetric photoreactions using flow microreactor. Organic photochemical reactions are highly effective methods forming complicated or distorted structures that cannot be obtained easily by conventional thermal reactions. On the other hand, photoreactions suffer from low reaction efficiency in common batch reactors owing to low light penetration. To solve this problem, microreactors have recently been fascinated. The asymmetric [2+2] photocycloadditions performed in both hand-made capillary reactors and purchased microreactors, and could be achieved higher diastereoselectivity and productivity than those in batch reactors. Additionally, the Paternò-Büchi type photoreactions in slug flow conditions with organic layer and unreactive layer (water or nitrogen gas) showed the unprecedented enhancement on reaction efficiency. These results demonstrate the good potential of micro flowreactor to achieve the efficient organic photoreactions not only in the lab scale chemistry but also in the process chemistry.