A mathematical model for the performance of the planar solid oxide fuel cell (SOFC) stack is described. The model considered the electric contact resistance between the electrode and interconnect rib. the gas transport in the electrodes, electronic and ionic conductions in the membrane-electrode assembly and the electrochemical reactions at the gas-electrode-electrolyte three phase boundaries. The model is capable of describing in detail the rib effect on the gas transport and the Current distribution in the fuel cell. The contact resistance is found to be an important factor in limiting the SOFC performance. Based on the interplay of the concentration and ohmic polarizations, numerical results are provided for the optimal rib widths for different pitch sizes and different area specific contact resistance (ASR(contact)). The optimal rib width is found to be linear to the pitch width for a given ASR(contact) and the parameters for the linearity are given. The parameters are little affected by the hydrogen concentration and the thickness, porosity or conductivity of the cathode. The influence of the cathode thickness on the SOFC performance is also examined. Contrary to the common belief oil the thin cathode (similar to 50 mu m), thicker cathode layer (100-300 mu m) is beneficial to the SOFC stack performance. (C) 2008 Elsevier B.V. All rights reserved.