Formation of amphiphilic poly(ethylene glycol)-b-polylactide (PEG/PLA) block copolymers was accomplished by using potassium alkoxides to initiate the anionic polymerization of ethylene oxide, with the living chain end initiating the polymerization of lactide. By using potassium 3,3-diethoxypropoxide as an initiator, block copolymers with an acetal moiety at the PEG chain end, which was later converted into an aldehyde group, were obtained. The amphiphilic block copolymers formed micelles in aqueous milieu. The conversion of acetal end groups to aldehyde groups was carried out by an acid treatment using 0.01 mol L-1 hydrochloric acid. The extent of the conversion attained was >90%, without any side reaction such as aldol condensation. The micellar structure may play an important role in preventing a possible aldol condensation between the neighboring two aldehyde groups at the PEG chain end. From dynamic light scattering measurements, no angular dependence of the scaled characteristic line width was observed in the case of the acetal-PEG/PLA(52/56) micelle, suggesting the spherical structure. The diameter and polydispersity factor of the polymeric micelle were influenced by the molecular weights and the composition of two components of the block copolymer. The block copolymer with the molecular weight of 5200 for PEG and 5600 for PLA was a most suitable balance for micelle formation with narrow distribution. Actually, the diameter and polydispersity factor (mu/Gamma(2)) of acetal-PEG/PLA(52/56),; determined by a cumulative method, were 33 nm and 0.03, respectively. No change in the micelle size and shape was observed before and after the conversion of the acetal end groups to aldehyde groups on the micelle. The critical micelle concentrations (cmc) of the polymeric micelle was 2-4 mg L-1, as determined by fluorescence spectroscopy using pyrene. This functionalized micelle, in particular the one carrying terminal aldehyde groups, is expected to have a wide utility not only in biomedical applications (e.g., drug delivery, diagnosis, and surface modification through the coupling of bioactive substances), but also for the construction of the supramolecular architecture.