Gambogic acid (GA) possesses good anti-tumor efficacy in preclinical studies, however, its poor hydrophilicity, short blood circulation time and side effect limited its clinical application. In this work, monomethyl poly(ethylene glycol)-poly(ε-caprolactone) (MPEG-PCL) copolymer was synthesized and used to encapsulate GA by a facile one-step solid dispersion and form nano-sized micelles (GA micelles). The GA micelles exhibited small average particle size (29±2 nm), high encapsulation efficiency (92.1±0.3%), and long drug release time-in vitro. Compared to free GA, GA micelles showed superior aqueous dispersity, better tumor cellular uptake, enhanced cytotoxicity and apoptosis induction effect against MCF-7 cells. Furthermore, in vivo studies demonstrated that GA micelles have better antitumor effect in the MCF-7 subcutaneous xenograft tumor model. Histopathological analysis of Ki-67 and TUNEL staining further proved that GA micelles could significantly suppress proliferation as well as increase the apoptosis of tumor cells. These results suggested that GA micelles could potentially improve therapeutic outcomes for breast cancer therapy.