The geometry of micro fuel cell design influences the reactants' mixing and the depletion at downstream of the channel and thus effects the cell performance. This paper proposes a design for membraneless micro fuel cells with an H-shaped cross-section and a small passage between the anode and cathode channels. The small passage restricts the mixing of the anode and cathode fluids in the main channel. Numerical simulations with electro-chemical reactions have been carried out to investigate the distribution and crossover of the reactants and also the mixing and depletion regions in the system. Results show that optimizing the size of the passage between the anode and cathode channels plays an important role in reducing the mixing of reactants and in increasing fuel utilization. The H-shaped design shows that the mixing region is reduced in size by about 20%, so the H-shaped design has 10 times less fuel crossover than the conventional rectangular design. Moreover, fuel utilization is increased by about 8% with respect to that of the conventional rectangular design. 90, angles between the passage and the anode and cathode channels provide the best layout for this H-shaped design. The aspect ratio 0.083 for the anode and cathode channels exhibits 23% higher fuel utilization than the conventional rectangular design. Moreover, the size of the passage has a significant influence on the boundary layer thickness, the depletion region and the current density. A micro fabrication of the H-shaped design was made and the open circuit voltages were measured. The results are compared with those in the available literature. (C) 2009 Elsevier Ltd. All rights reserved.