An amphiphilic diblock copolymer of poly(acrylic acid-b-DL-lactide) (PAAc-b-PDLLA) was synthesized byring-opening polymerization of DL-lactide initiated by hydroxyl-terminated polyacrylic acid (PAAc-OH).The critical micelle concentration (CMC) of PAAc-b-PDLLA in aqueous solution, determined by fluorescencespectroscopy using pyrene as a probe, was found about 80 mg L_1. A solution of PAAc-b-PDLLA intetrahydrofuran (THF) was dialyzed against pure water to form pH-responsive micelles. Transmissionelectron microscopy (TEM) measurement showed that the micelles exhibited regular sphericalmorphology and the diameters of particles were in the range from 40 to 90 nm. The micelles were stableat a pH above 3 or at an ionic strength below 1.0, however, they aggregated and precipitated in thesolutions when further decreasing pH or increasing ionic strength. Prednisone acetate, as a modelhydrophobic drug, was loaded into the polymeric micelles. In vitro release of prednisone acetate frompolymeric micelles showed that the release kinetics was strongly pH-dependent. Hydrophobic drugdisplayed "burst" release at pH 7.4, while only a small part of loaded drug released at pH 1.4. Thisprovides a new choice to design delivery system for the gastrointestinal tract (GI tract), where the pHenvironment is strongly acidic in stomach and basic in intestine. The cytotoxicity measurement by MTTassay indicated that PAAc-b-PDLLA was low toxic in HeLa cells with an IC50 value of 2.8 mg mL_1, whichsuggests that PAAc-b-PDLLA could be used as a safe candidate for pH-responsive drug delivery.