Narrowly size-distributed poly(styrene-co-methyl methacrylate) (poly(St-co-MMA))/SiO2 nanocomposite particles (NCPs) were efficiently prepared through miniemulsion polymerization by using a special use strategy of surfactants. A trace amount of sodium dodecyl sulfate (SDS) was post-added to monomer miniemulsions stabilized with polyoxyethylene (20) sorbitan monolaurate (Tween-20). The number fraction of the poly(St-co-MMA)/SiO2 NCPs reached 97% in the as-synthesized emulsion stabilized with 0.15 g of Tween-20 and 2.5 mg of the post-added SDS. The highly efficient formation of poly(St-co-MMA)/SiO2 NCPs was realized through controlled mergence between the monomer droplets and latex particles during polymerization. The influence of the amount of post-added SDS, surface hydrophobic modification degree of SiO2 nanoparticles, and SiO2 content on the particle properties of latex particles was systematically investigated. The amount of post-added SDS and surface hydrophobic modification degree of SiO2 nanoparticles are two determining factors for the efficiency of polymer/SiO2 NCP formation.