An analytical model is presented for the determination of the fractal dimensions of particles which are widely used as reinforcement in nanocomposites. The model is used to characterize the surface irregularity or roughness. It was found that fractal dimensions of both the contour and surface of particles depend only on the relative particles size ratio between secondary particles and subunits. It is proposed that, in practical applications, the fractal dimension of a certain reinforcement particle can be obtained by a combination of this model and a state-of-the-art instrument that can determine the sizes of primary and secondary particles by image analysis. It is possible to relate the fractal dimension with the adhesion and other physical and chemical properties at the interface between particles and matrix.