Biodegradable nano-particles such as polymeric micelles are very attractive for drug delivery device and other biomedical applications. Especially, polymeric micelles from AB-type diblock copolymers having amphiphilic structures through self-aggregation in aqueous solution are utilized for drug delivery device. Polylactide (PLA) is one of the representative biodegradable polymers and has good biodegradability, biocompatibility and mechanical property. Although many studies on PLA-based block copolymers have been reported, there has been little research done on polypeptide-PLA diblock copolymers. Polypeptides consisting of a-amino acids with functional (ionic) side-chain groups are hydrophilic polymer, and should be useful as hydrophilic and pH sensitive segments in the block copolymers. We synthesized poly(aspartic acid)-b-PLA (PAsp-b-PLA) and polylysine-b-PLA (PLys-b-PLA) through a polymerization of b-benzyl-L-aspartate-N-carboxy anhydride (Asp(OBzl)-NCA) or N-carbobenzyloxy-L-lysine-N-carboxy anhydride (Lys(Z)-NCA) with PLA having mono amino terminal (NH₂-PLA) as a macroinitiator. PAsp-b-PLA and PLys-b-PLA have amphiphilic structure consisting of a hydrophilic (negatively or positively charged) polypeptide segment and a hydrophobic PLA segment, creating ideal conditions for the formation of polymeric micelles. Furthermore, the poly(aspartic acid) and polylysine segment are functionalized with negatively charged carboxylic groups and positively charged anino groups which should result in pH dependent changes in secondary structure (ex: a-helix, random coil). It is well known that the size of polymeric micelles is affected by the ratio of the length of the hydrophilic and hydrophobic segments. In this study, several kinds of PAsp-b-PLA and PLys-b-PLA copolymers having various hydrophilic/hydrophobic chain length ratios were synthesized, and the relationship between the molecular structure of the copolymers and diameter of the polymeric micelles were investigated. In addition, the effects of pH on the PAsp-b-PLA and PLys-b-PLA polymeric micelles in aqueous solution were investigated. PAsp-b-PLA was synthesized according to the Scheme. The degree of polymerization of polypeptide segment could be easily controlled by changing the ratio of amino acid-NCA and NH₂-PLA. The obtained diblock copolymer was dissolved in DMSO and placed in a permeable dialysis membrane (MW cut off: 500). The permeable membrane was immersed in water and dialyzed. The obtained solution was centrifuged to separate little amount of precipitate from the supernatant. The supernatant was observed by dynamic right scattering (DLS) measurement. The DLS data revealed the formation of polymeric micelles by self-aggregation. The average diameter of the polymeric micelles was about several tens nm. DLS measurements of redispersed polymeric micelles in water after freeze drying were carried out. The average diameter of the redispersed polymeric micelles was the same as that after dialysis. This result indicates that the polymeric micelles can be stored in a dry state after freeze-dry. The pH dependence of the diameter of the polymeric micelle was investigated by DLS measurement. As expected the polymeric micelle showed pH sensitive change in diameter. The diameter changes were also confirmed atomic force microscopy (AFM) measurements. We successfully prepared biodegradable polymeric micelles composed of hydrophobic PLA segment and hydrophilic PAsp or PLys segment. The diameters of the polymeric micelles changed to correspond to the pH. These pH-sensitive polymeric micelles could be utilized as functional biodegradable drug carriers and other biomedical devices.