A new kind of drug-crosslinked polymer nanoparticle was prepared. The nanoparticles were composed of a phenylboronic acid modified hydroxycamptothecin crosslinker (HCPT-diHMPBA) and a 1,2-diol-rich poly(ethylene oxide)-b-poly(glycerol monomethacrylate) diblock copolymer (PEG-PGMA), and crosslinked by a phenylboronic ester bond. Dynamic light scattering measurement and electron microscopy observation revealed that the nanoparticles had a spherical morphology with a hydrodynamic diameter of 146 nm. The phenylboronic acid modified hydroxycamptothecin crosslinker could be released from the nanoparticles in a sustained manner and converted into pharmaceutically active hydroxycamptothecin (HCPT) in an enzyme-containing medium, as shown in the in vitro cytotoxicity test. Cellular uptake examination by confocal laser scanning microscopy showed that the nanoparticles were well internalized by cancer cells and active HCPT was found inside the cells.