20(S)-Protopanaxadiol (PPD), a ginsenoside isolated from Pananx quinquefolium L., has been shown toinhibit growth and proliferation in several cancer cell lines. The aim of this study was to evaluate its anticanceractivity in human breast cancer cells. MCF-7 cells were incubated with different concentrations of 20(S)-PPDand cytotoxicity was evaluated by MTT assay. Occurrence of apoptosis was detected by DAPI and AnnexinV-FITC/PI double staining. Mitochondrial membrane potential was measured with Rhodamine 123. The Bcl-2and Bax expression were determined by Western blot analysis. Caspase activity was measured by colorimetricassay. 20(S)-PPD dose-dependently inhibited cell proliferation in MCF-7 cells, with an IC50 value of 33.3 μM at24h. MCF-7 cells treated with 20(S)-PPD presented typical apoptosis, as observed by morphological analysisin cell stained with DAPI. The percentages of annexin V-FITC positive cells were 8.92%, 17.8%, 24.5% and30.5% in MCF-7 cells treated with 0, 15, 30 and 60μM of 20(S)-PPD, respectively. Moreover, 20(S)-PPD couldinduce mitochondrial membrane potential loss, up-regulate Bax expression and down-regulate Bcl-2 expression.These events paralleled activation of caspase-9, -3 and PARP cleavage. Apoptosis induced by 20(S)-PPD wasblocked by z-VAD-fmk, a pan-caspase inhibitor, suggesting induction of caspase-mediated apoptotic cell death.In conclusion, the 20(S)-PPD investigated is able to inhibit cell proliferation and to induce cancer cell death bya caspase-mediated apoptosis pathway.