In the present study, a full three dimensional, isothermal, and steady state model is proposed for a planar solid acid fuel cell (SAFC) that uses CsH2PO4-AAM (anodic alumina membrane) composite as an electrolyte. A fuel cell is used for the generation of electricity and consists of anode, electrolyte, and cathode. In this model, mass, momentum, electrical charge transport and electrochemistry equations were used to describe the fuel cell regions (flow channel, gas diffusion layer, and catalyst layer). By using finite element method in COMSOL MULTIPHYSICS (V. 3.4) software, the equations of the model were solved numerically. Finally, the results of this model were compared with the experimental data reported in the literatures (polarization graph) to validate the model. The results show a good agreement with the reported experimental data. The advantage of this model is the prediction of SAFC behavior in different operational and geometrical conditions, and the results would be useful for establishing guidelines for SAFCs' design and optimization. (C) 2018 Elsevier Ltd. All rights reserved.