In membraneless micro fuel cells, the mixing and depletion widths are major factors that determine the cell performance. Cells in which these widths are too large exhibit severely reduced fuel utilization and, hence, less electrochemical reaction especially in the downstream region of the channel. For cells with conventional rectangular geometry, increasing the aspect ratio reduces the mixing width but reduces the effective electrode area.This work proposes a trident-shaped geometrical design for membraneless micro fuel cells in which the anode fluid, cathode fluid and proton-conducting fluid are introduced through three distinct inlets. The anode and cathode fluids are interconnected by the proton-conducting fluid channel. In addition, the anode fluid and proton-conducting fluid are connected by a small narrow passage, and the cathode fluid and proton-conducting fluid channel are also connected by a small narrow passage. Numerical simulations, including the effects of electrochemical reaction and fluid flow, are carried out to investigate reactant distributions in the downstream region of the channel and to study fuel utilization. A fuel utilization of around 51% is achieved when the two opposite walls are used as reaction surfaces and the inlet velocity is set at 0.01 m s(-1). By varying the cell length and expanding the reaction Surface areas by including additional surfaces within the cell, simulations show that the fuel utilization can be improved to around 86%, which is much higher than has been achieved in previous studies. The present numerical results are validated by comparison with available literature data. (c) 2008 Elsevier B.V. All rights reserved.