A novel design of membraneless microfuel cell employing a planar microchannel has been investigated theoretically in this study. The fuel and oxidant are. respectively, formic acid and oxygen and both dissolved in dilute sulfuric acid solutions. Both liquid streams enter the planar microchannel and flow in parallel without the need of a membrane to separate them. A theoretical model is developed to simulate the species transport in both anode and cathode streams and the cell performance is analyzed accordingly by examining the effects of flow rate, concentration, and the geometric size of the system. The results show that the cell performance is mainly restricted by the high transport resistance in the cathode stream. It is found that the transport of oxygen to the cathode electrode can be improved significantly by using a higher flow rate or oxygen concentration, or a thicker cathode catalyst layer. However, the effectiveness of the flow rate and thickness of catalyst layer diminishes gradually which indicates that there exist optimal conditions of these parameters. The influences of thickness and length of the microchannel on cell performance are also examined in detail. (c) 2006 Elsevier Ltd. All rights reserved.