This paper presents an easy-to-approach mathematical model for planar type solid oxide fuel cells. It is developed to be used as a tool for students in fuel cell class to conduct parametric studies in order to grasp fundamentals and to understand the effects of mass transfer, electrochemical reaction, and current conduction on the voltage-current relationship in a fuel cell at different operating conditions. Using the model, the three over-potentials caused by different polarizations in a planar type solid oxide fuel cell can be identified and compared. The analysis for the mass transfer polarization is based on average mass transfer model analogous to an average heat transfer process in a duct flow. The effect of species' partial pressures on the exchange current density in activation polarization is counted in the activation polarization model. A discretized current conduction circuit is applied to analyze the ohmic losses from anode current collector to cathode current collector. The model is of significance to the design and optimization of solid oxide fuel cells in industrial application.