In this paper, the formulation and validation of a model for the prediction of the wear particles size in boundary lubrication is described. An efficient numerical model based on a well-established BEM formulation combined with a mechanical wear criterion was applied. The behavior of the model and particularly the influence of the initial surface roughness and load was explored. The model was validated using measurements of the wear particles formed in steel–steel and steel–brass contacts. In the case of steel–steel contact, a reasonable quantitative agreement was observed. In the case of steel–brass contact, formation of the brass transfer layer dominates the particles generation process. To include this effect, a layered material model was introduced.