A flexible catalytic microreactor was developed on the basis of a new concept of element assembly. The proposed reactor consists of elements of various catalysts with large surface area and reactor elements fabricated separately, and the microspace is created by optionally assembling them. A microreactor, assembled of 21 sets of elements, was used to decompose methanol for selectively producing hydrogen. At a reaction temperature of 280 degrees C, methanol conversion and hydrogen yield successfully reached 93% and 1.48, respectively. Conversion increased with decreasing size of the microchannel, keeping the ratio of H-2 to CO at ca. 2. This clearly shows that the reaction in the microchannel is advantageous to the conversion and selectivity compared with those reached in a conventional fixed bed reactor.