Polymer nanoparticles with phase-separation structures can be prepared by simple solvent evaporation process from block copolymer or polymer blend solution (self-organized precipitation (SORP)). In this report, core-shell organic-inorganic nanocomposite particles were prepared by combination of block copolymer stabilizing Au nanoparticles and polymer blend. Block copolymer stabilizing Au NPs were prepared by reduction of Au ion in their micelles. Nanocomposite particles of poly(methyl methacrylate) (PMMA) and block copolymer stabilizing Au NPs are successfully prepared by using the SORP method.