The aim of this study was to explore whether the ethanolic extract of Ardisia gigantifolia rhizomes (AGB-5), a traditional herbal medicine from China, could affect the proliferation of human breast adenocarcinoma (MCF-7) cells in vitro and to explore the antitumor effects of AGB-5 in BALB/c mice engrafted with MCF-7 cells. The results showed that AGB-5 markedly inhibited the proliferation of MCF-7 cells with an IC50 value of 11.89±1.12 µg/mL, increased the S phase and decreased the G2/M phase without influence on G1 phase. MCF-7 cells treated with AGB-5 presented a dose-dependent increase of apoptosis compared with the control group. AGB-5 also significantly increased the activity of caspase-3 and -9 in a dose-dependent manner in MCF-7 cells. Furthermore, in an in vivo model, AGB-5 reduced tumor volume, brought back the red blood cell (RBC) and white blood cell (WBC) count near to normal value, enhanced superoxide dismutase and catalase level of MCF-7 bearing mice. This is the first study to verify the antitumor activity of A. gigantifolia in vivo. The results suggest that AGB-5 may have potential beneficial effects against human breast adenocarcinoma.