Direct internal reforming solid oxide fuel cells (DIR SOFCs) have complicated distributions of temperature and species concentrations due to various chemical and electrochemical reactions. The details of these properties are studied by a 3-D numerical simulation in this work. The simulation modeling used governing equations (mass, momentum, energy and species balance equations) generally suitable to porous medium with porosity variable of zero (solid), 0.3 (porous medium) and 1.0 (fluid). Chemical kinetics equations for the internal reforming and shift reactions based on the Langmuir-Hinshelwood model were incorporated. Hydrogen and carbon monoxide oxidations were considered both participating in electrochemical reactions. The experimentally measured current density potential curves were compared with the simulation data to validate the code, which revealed that the simulation model was able to predict the dilution effect of nitrogen and the mass transfer under high current densities. It is found that the temperature dramatically declined near the fuel inlet with strong endothermic reactions, but it increased along the fluid flow with electrochemically exothermic reactions. A low steam-to-carbon ratio (SCR) led to high steam reforming and water gas shift reaction rates, which generated a greater amount of hydrogen. Therefore, current density increased with low SCR. The average current density due to carbon monoxide electrochemical oxidation varies from 205.3 A/m(2) under an SCR of 2.0 to 47.6 A/m(2) under an SCR of 4.0. The average current density due to hydrogen electrochemical oxidation was 5535.4 A/m(2) under an SCR of 2.0, which was 27 times higher than that of carbon monoxide. The total current density ranged from 5740.8 A/m(2) under an SCR of 2.0 to 2268.9 A/m(2) under an SCR of 4.0. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.