Breast cancer is the most vicious killer for women,and tumor metastasis is one of the leading causes of breast cancer therapy failure.In this study,a new pH-sensitive polymer (polyethylene glycol-block-poly[(1,4-butanediol)-diacrylate-β-N,N-diisopropylethylenediamine],BDP) was synthesized.Based on BDP,docetaxel/silibinin co-delivery micelles (DSMs) was constructed.DSM had a well-defined spherical shape under the transmission electron microscope with average hydrodynamic diameter of 85.3±0.4 nm,and were stable in the bloodstream but could dissociate to release the chemotherapeutic agents in the low pH environment of the endo/lysosomes in the tumor cells.Compared with free drugs,DSM displayed greatly enhanced cellular uptake,higher cytotoxicity and a stronger antimetastasis effect against mouse breast cancer cell line 4T1.In 4T1 tumor-bearing mice treated with DSM (twice a week for 3 weeks),the inhibition rate on tumor growth and metastasis reached 71.9％ and 80.1％,respectively.These results reveal that DSM might be a promising drug delivery system for metastatic breast cancer therapy.