We describe a multiphase formulation to study numerically the propagation of a line fire in a forest fuel bed. One of the objectives of these studies is the improvement of knowledge on the fundamental physical mechanisms that control the propagation of forest fires. In complement of the experimental approach, this simulation tool can also be used for the development of simplified operational models used for instance for the prediction of the rate of spread (ROS) of wildland fires. The decomposition of solid fuel constituting a forest fuel bed as well as the multiple interactions with the gas phase are represented by adopting a multiphase formulation. This approach consists in solving the conservation equations (mass, momentum, energy) averaged in a control volume at a scale sufficient to contain several solid particles in the surrounding gas mixture. After a presentation of the equations and closure sub-models used in this approach, some numerical results obtained for the propagation of a line fire in a pine needles litter are presented and compared with experimental data obtained in laboratory. These results show that the rate of spread of fire in the fuel bed is primarily controlled by the radiative heat transfer. By increasing the fuel load (with a constant packing ratio), the results show the existence of two modes of propagation. A first area where the ROS varies linearly with the fuel load followed of a second where the ROS becomes independent of the load. By introducing the optical thickness characterizing the fuel bed, this difference in mode of propagation was interpreted like the demonstration of two modes of radiative transfer (optically thin and thick, respectively). The analysis of the distributions of the mass fractions of fuel and oxidant present in the gas mixture integrated through the depth of the fuel bed shows that the propagation velocity could also be limited by the lack of oxygen or fuel available in the ignited zone to maintain the pilot flame.