In this paper, a transient heat transfer model to simulate the heat-up and start-up periods of co- and counter-flow direct internal reforming solid oxide fuel cells is developed and presented. In this comprehensive model, all the heat transfer mechanisms, i.e. conduction, convection, and radiation, and all the polarization nodes, i.e. ohmic, activation, and concentration, are considered. The heat transfer model is validated using the results of a benchmark test and two numerical studies obtained from the literature. After validating the model, the heat-up, start-up, and steady-state behaviors of the cell are investigated. In addition, the first principal thermal stresses are calculated to find the probability of failure of the cell during its operation. The results of the present model are in good agreement with the literature data. It is also shown for the given input data that counter-flow case yields a higher average current density and power density, but a lower electrical efficiency of the cell. For the temperature controlled heat-up and start-up strategy, the maximum probability of failure during the operation of the cell is found to be 0.068% and 0.078% for co- and counter-flow configurations, respectively. (C) 2009 Elsevier B.V. All rights reserved.