Polystyrene (PSt)-based particles are versatile components for composite materials, and their sizes and surface properties play a vital role in composite processes. In this study, we developed highly useful PSt-based particles by soap-free emulsion polymerization. The size and surface properties of monodisperse PSt-based particles were controlled using an ionic comonomer of sodium p-styrenesulfonic acid (NaSS) and a nonionic comonomer of N-isopropylacrylamide (NIPAM). The particle sizes decreased with increasing NaSS concentration. A weak dependence of particle sizes on NIPAM concentration was observed at a fixed NaSS concentration of 2 mM. The particles synthesized at an NIPAM ratio of 10 mol% or more exhibited shrinking behavior at 50°C, although an increase in NaSS concentration suppressed the shrinking behavior of the particles. Compared to the results of previous reports, the ratio of the nonionic (NIPAM) and the ionic (NaSS and initiator) components corresponds to the shrinking degree of PSt-based particles copolymerized with NIPAM. Surface tensions of the suspensions were measured to evaluate the surface properties of the PSt-based particles because poly(N-isopropylacrylamide) (PNIPAM) has an intrinsic property of surface activity. The surface tension decreased with increasing NIPAM ratio up to 10 mol%. An analysis using a fluorescence probe of Laurdan experimentally indicated that the addition of NIPAM formed a localized hydrophobic environment on the particle surfaces, resulting in surface activity. These results suggest that the employments of ionic comonomer and NIPAM enable independent controls over sizes and surface activities of PSt-based particles, which is required for development of new functional materials incorporating monodisperse polymer particles as building blocks.