A polarization model for a solid oxide fuel cell (SOFC) with a bi-layer electrolyte is developed and a set of explicit expressions for output current, maximum power and thickness ratio of bi-layer electrolyte are obtained. In the model, two dominating polarizations, activation polarization and ohmic polarization, are considered and the concentration polarization is neglected. The activation polarization at the electrodes is described by the Butler-Volmer equation. The ohmic polarization is represented by the charge transport equation, and this equation is solved with the assumption of constant ionic conductivity. To close the model equations, a supplemental relation is introduced by the energy conservation analysis. The modeling results compare well with the experimental data. The model is further used to study the effect of cathode exchange current density on the working characteristics of a solid oxide fuel cell with a bi-layer electrolyte. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.