Emulsifier-free emulsion polymerization of styrene in a water/acetone mixture under microwave irradiation resulted in narrowly distributed stable polystyrene nanoparticles with an averaged hydrodynamic radius (R-h) down to 35 nm when 50 wt % of acetone was added. For a given initiator potassium persulfate (KPS) concentration, (R-h) was proportional to the monomer concentration in the range 1.2 - 7.0 wt %; while for a given monomer concentration, (R-h) showed a minimum with an increasing initiator concentration. After both the roles of stabilization and destabilization of the initiator (KPS) were considered, we introduced a new parameter, the effective surface area stabilized per ionic group, and reformulated the Wu plot of (R-h) proportional to W-monomer/W-stabilizer into (R-h)/[k(1)(1 + k(2)W(KPS))] proportional to W-monomer/W-initiator. It can qualitatively explain both the monomer and initiator concentration dependence of the particle size over a wide concentration range. The study of the salt effect showed that (R-h) increased with the sodium sulfate concentration for fixed KPS and styrene concentrations.