Abstract: A pH-sensitive amphiphilic polymer Poly(ethylene glycol)-phenylborate-poly(ε-caprolactone) (mPEG-PBE-PCL) linked by a borate structure was prepared by ring-opening polymerization of ε-caprolactone (ε-CL) initiated by a benzene-borate-containing polyethylene glycol macromolecule (mPEG-PBE-OH). The polymer was capable of self-assembling into a "core-shell" nanomicelles structure in aqueous environment, which can load doxorubicin (DOX) in their cores. The structure of polymer was characterized by ¹H-NMR, IR spectra and gel permeation chromatography (GPC). The morphology and particle size of the prepared micelles were determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS). The drug loading and loading efficiency were analyzed by ultraviolet absorption detection. The investigations on pH-sensitive release properties of the micelles and the in vitro cytotoxicity were carried out as well. The results showed that the polymer micelles had a low critical micelle concentration, showed spherical micelles with a diameter of about 127 nm, and had a high loading capacity for DOX. Furthermore, the polymer presented good pH responsiveness and biocompatibility. The loaded DOX was released in weak acidic environment of tumor cells and was delivered effectively to nucleus. DOX@mPEG-PBE-PCL had a similar antitumor activity with free DOX·HCl against murine melanoma B16F10 cells. Therefore, such smart micelles containing pH-responsive boronate bonds present great potential in efficient intracellular delivery of antitumor drugs.