Diffusion bonding is a solid-state welding process that allows the joining of similar or dissimilar heterogeneous materials preventing the weldability problems associated to fusion welding. Modelling of this process has been attempted by several authors like Hill and Wallach [1]. These authors considered that the plastic deformation between two surfaces in contact is one of the most important mechanisms involved in the process. This paper reports the results of diffusion bonding of 1045 steel. Modelling of the process was done using Hill and Wallach [1] parameters to define the surface roughness condition. During preparation for diffusion bonding a series of long parallel ridges, typically with 0.2 to 2 µm high (roughness asperity height) and 30-70 µm width (roughness wavelength) were produced. The initial contact of the asperities on the prepared surfaces created a series of voids with an elliptical shape (infinitely long parallel cylinders with elliptical cross-section). During the diffusion bonding process the shape of the voids changes, becoming smaller, mainly due to the pressure application. Since this is a diffusion controlled process, the use of temperature promotes the deformation process. A model using Finite Element Analysis coupled to a commercial software was developed considering two surfaces, containing an half void, which were brought into contact. The first model was developed with only one elliptical void with two different widths (30 and 70 µm) and 2 µm high. In this model enough pressure was applied to close completely the void. The second model considered three voids placed together to simulate the voids continuity. Finite Element Analysis was developed considering that initial contact between surfaces with asperities creates a series of voids with elliptical shape. In the initial stage of the process, the pressure applied changes the shape of the voids due to plastic deformation increasing the contact area. Microstructures were investigated.