pH-responsive polymer shell chitosan/poly (methacrylic acid) (CS-PMAA) was coated on mesoporous silica nanoparticles (MSN) through the facile in situ polymerization method. The resultant composite microspheres showed a flexible control over shell thickness, surface charges and hydrodynamic size by adjusting the feeding amount of MSN and the molar ratio of [–NH2]/MAA. The MSN/CS-PMAA composite microspheres were stable in the pH range of 5–8 as well as in the physiological saline (0.15 M NaCl). Doxorubicin hydrochloride (DOX) was applied as a model drug to investigate the drug storage and release behavior. The results demonstrated that DOX could be effectively loaded into the composite microspheres. The cumulative release of DOX-loaded composite microspheres was pH dependent and the release rate was much faster at low pH (5.5) than that of pH 7.4. The cytotoxicity test by MTT assay showed that the blank carrier MSN/CS-PMAA microspheres were suitable as drug carriers. The cellular uptake of composite microspheres was investigated by confocal laser scanning microscopy (CLSM), which indicated that MSN/CS-PMAA could deliver the drugs into HeLa cell. The above results imply that the composite microspheres are a promising drug delivery system for cancer therapy.