This study examined the temperature distributions of the anode and cathode gases of the cell body as well as the current density distributions at each point of the direct internal reforming molten carbonate fuel cell (DIR-MCFC) using numerical modelling. The model was based on assumptions and experimental data from a 5cm x 5cm sized unit cell operation. The results showed there was an approximately 13 degrees C temperature difference between the initial point (0, 0) and end point (1, 1) of the cell body and the temperature increased steadily along with the direction of the anode gas flow. The temperature distribution of the anode gases showed a similar trend to those of the cell body. The temperature of the anode gases was an average I VC lower than that of the cell body. The temperature distributions of cathode gases were relatively higher than those of the anode gases and the cell body. The temperature distributions at each point of the cell body, including the anode and cathode gases. could be explained by the different rates of the electrochemical, methane steam reforming and water gas shift reactions at each point in the cell body. The current density distribution at the entrance of the cell was the highest at 290 mA cm(-2). and decreased steadily to 150 mA cm(-2) at the exit. These results were also confirmed by the amount of hydrogen reacted in the electrochemical reaction (referred to Part II). Finally, modelling simulations showed a non-uniform distribution of the temperature and current density throughout the DIR-MCFC were observed. In addition, it was confirmed that the distributions of the reaction rates and gas compositions at each point of the cell also showed a great deal of difference throughout the DIR-MCFC. The non-uniformity of these temperature distributions can lead to deterioration in the cell performance. These might provide the necessary information for solving these problems. Copyright (c) 2005 John Wiley & Sons. Ltd.