Doxorubicin hydrochloride (DOX·HCl),an anthracycline antibiotic,is used in the first line treatment for a wide range of cancers.In order to reduce the side effects of free DOX·HCl,many nanocarriers have been developed.We have already developed a green and convenient method for the encapsulation of DOX·HCl through the electrostatic interaction with methoxy poly(ethylene glycol)-block-poly(L-glutamic acid) (mPEG-b-PLG) in aqueous solution [1].However,the DOX·HCl-loaded mPEG-b-PLG micelles [DOX(L)] are unstable in blood circulation,which results in a fast release of free DOX·HCl after administration Therefore the blood circulation time of the DOX(L) is comparable to that of free DOX·HCl.