INTRODUCTION AND OBJECTIVES: Breast cancer is the second most common type of cancer in the world, and the one responsible for most deaths in women. The process of carcinogenesis comprises the initiation, promotion and tumor progression, and it has been shown that natural products may act at these different stages. Oxyresveratrol (trans-2, 3’, 4, 5’-tetrahydroxystilbene; OXY) is a polyphenol found mainly in the Morus alba L. belonging to the family Moraceae and has several biological effects describes as leishmanicidal, anti-oxidant and anti-inflammatory. The aim of this study was to investigate the effect of OXY in cell death of in MDA-MB-231 breast cancer cell. MATERIAL AND METHODS: The phases of the cell cycle were analyzed in flow cytometry after labeling with RNAse-PI. The analysis of apoptosis and necrosis were conducted with annexin V-FITC and PI by flow cytometry and the confirmation of apoptotic cells was done using DAPI staining. To determine the effect of OXY on MDA-MB-231 cells, caspase-3 involved in apoptosis was analyzed by Western Blot, Immunocytochemistry and real-time quantitative polymerase chain reaction (qPCR). RESULTS AND CONCLUSION: Our results showed that OXY presented cytotoxicity in MDA-MB-231 cells with IC50 of 287.08 µM after 24 hours of treatment. To further examine the cytotoxic effects of OXY over a prolonged period of time, clonogenic assays were performed cell for 18 days after treatment with IC50. Our data showed that the clonogenic ability of MDA-MB-231 cells was inhibited in the presence of 287.08 µM of OXY. The OXY increased by 3-fold the percentage of annexin-V positive cells, which characterizes apoptosis death. To further confirm the mode of cell death, the MDA-MB-231 cells were assessed by DAPI staining. Our results demonstrated that after incubation with 287.08 μM OXY for 24 hours, the apoptotic cells increased 2.48-fold. Our immunocytochemistry results showed that OXY increased 3.47-fold the levels of caspase-3. In addition, OXY induced significant upregulation of caspase-3 demonstrated by RT-PCR and Wester Blot results. In summary, our results demonstrated the in vitro anti-breast cancer effect of OXY, which may suggest an important candidate for the source of new molecules to be used in anticancer therapy.