Abstract: Surface of nano-SiO₂ particles was covered with PMMA by emulsion polymerization to prevent their reunion. The structures of emulsion polymeric products of alkyl-nano-SiO₂/MMA were characterized by TEM and FTIR. The results indicate that the particles of emulsion polymeric products are approximate ball in shape and that the particle consists of centered core(SiO₂) and shell (PMMA) and that chemical bonds exist between the core and shell of alkylaed-nano-silica/PMMA. The toughness, process flowability and heat resistance of PC were notably raised by core-shell particles of alkylated-nano-silica/PMMA with the cooperation of a third component.Its toughening mechanism was studied with the newest theory of multi-layer crack extension.