There is a lack of investigation on the effects of hydrophilic comonomer and pH on the particle size of functionalized polystyrene colloids. In this study, polystyrene (PS) colloids were prepared via emulsifier-free emulsion copolymerization of styrene (S) with 2-(dimethylamino) ethyl methacrylate (DMA) and methyl methacrylate (MMA) at pH 3.0 and 7.0. The sizes of the colloidal particles greatly depended on the comonomer type and pH. The hydrodynamic diameters of the particles ranged from 95 nm to 265 nm and their polydispersity index (PDI) values were less than 0.1. The particles containing comonomer moieties had spherical morphology and good stabilities. P(S/DMA) particles, especially showed excellent colloidal stability and had positive zeta potential values at the acidic pH range as a result of protonation of tertiary amine group of the DMA moiety. In addition, DMA functionalized PS colloids enabled to easy way for the preparation of PS/SiO2 composites. By physically deposition of silica nanoparticles onto the P(S/DMA) at acidic medium, PS/SiO2 core/shell composite particles were obtained. Both preparations of PS colloids and PS/SiO2 composites were easy and one-pot syntheses.