To evaluate the efficacy of doxorubicin (DOX) loaded micelles in enhancing DOX radiosensitivity in DOX-resistant K562 tumor cells (K562/DOX cells), DOX loaded polyethylene glycol-polycaprolactone (PEG-PCL) copolymer micelles and pluronic 105 (P105) micelles, and composite micelles composed of PEG-PCL and P105 were prepared. By using MTT assay, soft agar cloning assays, confocal laser scanning microscopy, and flow cytometry analyses to evaluate the radiosensitivity of each compound, DOX loaded micelles were found to increase the radiosensitivity of K562/DOX cells, as revealed by a marked cellular uptake and its sustained, slower release than free DOX. The micelles encapsulating DOX significantly enhanced its cytotoxicity in K562/DOX cells. Combined treatment with the encapsulation of DOX in micelles and radiotherapy therefore warrants investigation in clinical trials as a potential anticancer strategy with increased efficacy and reduced side effects.