Objective: To investigate the cytotoxic effect of ethanolic extract of the leaves of asam kandis (Garcinia cowa Roxb.) against T47D breast cancer cells. Methods: The cytotoxicity of ethanol extract was carried out by measuring the activity of mitochondrial dehydrogenase in living cells that have ability to convert dissolved MTT pale yellow to purple formazan product. The extract was added at various concentrations (0.1, 1, 10 and 100 μg/ mL). The level of cytotoxicity was determined by calculating the IC50 value that was based on the percentage of the cell death after 24 hours treatment with the extract. Cell morphological changes were observed by using inverted microscope. Results: The IC50 value showed that ethanol extract of leaves of asam kandis could resist T47D breast cancer cells with IC 50 6.13 ± 3.51 µg/mL. The statistic results proved that ethanol extract of the leaves of asam kandis could inhibit the growth of T47D breast cancer cells significantly at concentrations of 10 μg/mL and 100 μg/mL. Conclusion:The results suggest that ethanol extract of the leaves of asam kandis was potential source of herbal medicine for cancer-related ailments. Key words:Asam kandis, Beast cancer, Cytotoxicity, Garcinia cowa Roxb, MTT Assay, T47D. SUMMARY• The IC 50 value of the ethanolic extract of the leaves of asam kandis (Garcinia cowa Roxb) is 6.13 ± 3.51 µg/ml towards T47D breast cancer cell line. The results suggested that ethanol extract of the leaves of G. cowa was potential source of herbal medicine for cancer-related ailments. PICTORIAL ABSTRACTAbbreviations used: MTT: [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide], IC: Inhibition of Concentration.