Flocculation of fines and fillers is important in the papermaking industry. To fulfill the need for accurate control of flocculation, laser diffraction spectroscopy (LDS) was selected to supply the floc size distribution. LDS allowed the detection of different flocculation mechanisms depending on the flocculant characteristics. The analysis relied mainly on the collection of two parameters: the size distribution of the flocs, characterized by its median, and their fractal dimension. Floc resistance to shear was assessed and related to the floc structure through the analysis of the fractal dimension. For the lower charge density, flocs grow faster, becoming less compact and, thus, less resistant. LDS proved to be a valuable technique to monitor flocculation processes. However, to obtain significant results, a close control of obscuration in the measuring cell is required.