This paper reports a theoretical investigation on the orientation-dependence of the performance of membraneless microfluidic fuel cell (MMFC) and the relevant gravity effects. A coupled computational fluid dynamics (CFD)/electrochemical model is used for the analysis. The results show that gravity significantly affects the hydrodynamics in the microchannel and consequently reduces the cell performance dramatically with certain orientations. This important phenomenon has been widely ignored in previous works. Parametric analysis offers useful insights for enhancing the MMFC designs that minimize the adverse effects towards a feature of orientation-independence. (C) 2011 Elsevier Ltd. All rights reserved.