A comprehensive numerical study of membraneless micro fuel cells with various geometries is carried out with the aim of reducing the mixing of the anode and cathode fluids and increasing their fuel utilization. Designs with blocks or obstacles in the main channel or a main channel with a wavy shape result in very little improvement in these properties or even in their deterioration. However, some designs with other types of channel cross-section exhibit much less mixing of the two fluids in the main channel. In particular, an octagonal cross-section is found to result in better performance. However, the difficulty of the micro fabrication of fuel cells with this design encourages us to test two other geometries for the cross-section: H-shaped and trident-shaped. The H-shaped cross-section exhibits much less mixing in the main channel and much higher fuel utilization. The fuel cell with a trident-shaped cross-section has two inlets for the anode and cathode fluids and a third inlet for the proton-conducting fluid, and is found to be the best design in that the anode and cathode fluids are more restricted to their respective electrodes (reaction surfaces). Further, in this design the reactants cover only 40% of the channel width, which is much less than in the other designs, and maximum fuel utilization is obtained. The failure and success cases will guide for future geometrical design of any micro fluidic devices where mixing and non-mixing issues are the major concerns. The present numerical results are validated by comparison with literature data.