The present invention discloses the substrate and the micro reactor for mixing two kinds of fluids. The micro reactor of the present invention comprises a housing having first and second inlet ports and an outlet port formed thereon; and a plurality of substrates stacked in the housing, wherein the substrate has a space formed at a central portion thereof in one direction; a plurality of first channels extended from one side thereof to the space and corresponding to the first inlet port; and a plurality of second channels extended from the other side thereof to the space and corresponding to the second inlet port; wherein a portion between two neighboring first channels corresponds to the second channel and a portion between two neighboring second channels corresponds to the first channel to form sequentially reaction interfaces of the first and second fluids in the space.