Icariside II (ICA II) is a bioactive flavonoid in the traditional Chinese medicine Epimedii Folium with potential anticancer effects. However, poor water solubility and low systemic bioavailability limit its application in clinic. Therefore the present study aimed to develop an injectable polymeric micellar system delivering ICA II for cancer chemotherapy. The amphiphilic block copolymers were used as drug carrier, and the drug loaded micelles were prepared via thin-film hydration according to the optimal formulation and conditions from the response surface methodology. The physicochemical properties of micelles were characterized and the antitumor effects were investigated by the in vitro and in vivo assays. Resultantly the micelles had an average particle size of about 20 nm along with a narrow size distribution, and favorable drug loading (∼10%) and entrapment efficiency (>98%). The drug loaded micelles displayed sustained drug release over 10 days in the in vitro system. Compared with the free drug of ICA II, these micelles showed significantly enhanced cytotoxicity against in vitro proliferation of human hepatocellular carcinoma SMMC-7721 cells. The results from nude mice bearing human liver carcinoma further revealed the potent anticancer efficacy in vivo of these micelles. All the findings thus demonstrated that the micellar formulation of the natural bioactive flavonoid ICA II may serve as promising nanocarrier for anticancer treatments.