Disclosed herein is a membraneless micro fuel cell. A cathode fluid and an anode fluid with a low Reynolds number flowing along a cathode channel and an anode channel are formed to have an interface with each other through a micro passageway and to be mixed by only diffusion so that the direct mixing of the cathode fluid and the anode fluid is prevented, making it possible to prevent reactants from being depleted at an electrode surface as well as to increase the efficiency of the fuel cell.