This paper numerically investigates a portable fuel processor, i.e., micro-scale auto-electrolytic cell (AEC), for on-site hydrogen production in an economical, spontaneous and controllable manner. The AEC in this study consists of a galvanic couple of magnesium and steel in sodium chloride solution. A single Laplace's equation with boundary conditions determined from electrode reaction kinetics is solved for the potential inside the AEC. A dynamic mesh model based on arbitrary Lagrangian-Eulerian description is applied to track the moving boundary of the dissolving magnesium anode. Based on the model, the spatio-temporal distributions of potential, current density, hydrogen generation rate and other important parameters associated with the AEC are obtained. A great enhancement of hydrogen generation rate is found achievable by miniaturizing the AEC. In addition, parametric analyses are also performed focusing on important geometric factors. The study suggests that it would be better to arrange a number of micro-scale AEC units together to attain a desired total hydrogen output. The present study contributes to a better understanding of the hydrogen generation characteristics of AECs, hence facilitates their future development. The developed model can also serve as a useful tool to study other similar electrochemical systems. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.