Amphiphilic poly(ethylene glycol)-b-polylactide (PEG/PLA) copolymers with an aldehyde group at one end and a methacryloyl group at the other chain end were synthesized by anionic polymerization. The efficiencies of the functionalization at both ends were almost quantitative. The amphiphilic block copolymers formed micelles in aqueous media. Acetal groups on the micelle! surface were quantitatively converted to aldehyde groups by an acid treatment. The end methacryloyl group located in the core of the micelle was polymerized effectively to form core-shell-type nanoparticles having reactive aldehyde groups on the surface. The, size of the reactive nanoparticle was 20-30 nm which was constant with temperatures up to 60 DC. The stability of the micelle was also confirmed by a sodium dodecyl sulfate (SDS) treatment. When SDS was added to the nanosphere solution to 20 mg/mL, the particle was not collapsed. The particle was stable enough even in organic solvents. This functionalized micelle having high stability is not only expected to have wide utilities in biomedical applications (including drug delivery, diagnosis, and surface modification through the coupling of bioactive substances) but also to be of great interest as a novel supramolecular architecture.