Core-polystyrene/shell-silica nanocomposite particles are synthesized by simple soap-free emulsion polymerization employing positively charged silica sol. The polymerization is initiated with conventional anionic KPS. It is found that the silica sol should be added after the initiation and nucleation of the soap-free emulsion polymerization in order to obtain colloidally stable composite particles. The excess amount of silica promotes the formation of secondary particles, giving a broader particle size distribution. Above 10 wt% silica relative to styrene, the dispersion stability of the composite particles in water is significantly improved, showing a reduced sedimentation rate. The silica particles of average diameter 25 nm form almost a monolayer shell of approximately 30 nm thickness on the polystyrene particles. The incorporation of silica particles also gives rise to the enhancement of thermal stability when the silica concentration exceeds 10 wt% due to the strong interaction between silica and polystyrene molecules.