The details of the heat/mass transfer in a planar type solid oxide fuel cell that controls the energy conversion performance are studied by employing a three-dimensional numerical computation for the fields of velocity, gas mass fractions and temperature. The SOFC under investigation is a unit working in a SOFC stack. It has the tri-layer of anode-electrolyte-cathode and inter-connects having multiple channels for fuel and air. Two designs of the tri-layer, anode-supported and electrolyte-supported, are studied. Pre-reformed fuel gas with components of H 2, H 2O, CO, CO 2 and CH 4 is arranged in cross-flow direction with airflow. Further reforming and shift reaction in fuel channels were considered at chemical equilibrium. It was found that the consumption and production of gas species are different in the different channels. High current density was located in the upstream area of fuel channels. The operation conditions of current density affected the temperature level significantly.