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This work investigated the synthesis, characterization, and evaluation of surface-modified cationic silica nanoparticles as part of retention or flocculation systems. Quaternary amine groups were introduced onto the fumed silica surface using the cationic reagent (2,3-epoxypropyl)trimethylammonium chloride under various reaction conditions. To characterize the surface properties of the modified silica particles, the ζ potential and charge density were determined. Dynamic flocculation experiments showed that, when used alone, the cationic silica nanoparticles contributed little to inducing clay flocculation. However, when the nanoparticles were used in conjunction with an anionic polymer of high M w and low charge density, significant improvement in the flocculation of fine clay particles was achieved. The interaction between the cationic silica nanoparticles and anionic polymer, as well as the silica adsorption behavior on clay surfaces, was explored. The mechanism of flocculation induced by the dual-component system consisting of the cationic silica nanoparticle and anionic polymer was discussed.