Core-shell structured particles, which comprise the rubbery core and glassy layers, were prepared by emulsifier-free emulsion polymerization of poly(n-butyl acrylate/methyl methacrylate)/polystyrene [P(n-BA/MMA)/PS]. The particle diameter was about 0.22 mum, and the rubbery core was uncrosslinked and lightly crosslinked, respectively. The smaller core-shell structured particle-toughened PS blends were investigated in detail. The dynamic mechanical behavior and observation by scanning electron microscopy of the modified blend system with core-shell structured particles indicated good compatibility between PS and the particles, which is the necessary qualification for an effective toughening modifier. Notched-impact strength and related mechanical properties were measured for further evaluation of the toughening efficiency. The notched-impact strength of the toughened PS blends with uncrosslinked particles reached almost sixfold higher than that of the untoughened PS when 15 phr of the core-shell structured particles was added. For the crosslinked particles the toughening effect for PS was not obvious. The toughening mechanism for these smaller particles also is discussed in this article. (C) 2003 Wiley Periodicals, Inc.