The aims of this study was to isolate compounds from the methanol extract of stem bark of Garcinia cowa and to evaluated their cytotoxic activity against breast (MCF-7) and lung (H-460) cancer cell lines. The ethyl acetate fraction was separated by successive silica gel column chromatography to give three compounds. Based on spectroscopic comparison with those of the literature, these compounds were elucidated as 6-hydroxycalabaxanthone (2), 2-(3-methyl-2-butenyl)-1,5,6-trihydroxy-3-methoxy-4-(1,1-dimethyl-2-propenyl)-9H-xanthen-9- one(3), rubraxanthone (4), a-mangostin (5), a new compound 1,3,6-trihydoxy-7-methoxy-4-(4-acetoxy-3-methyl-2- butenyl)-8-(3,7-dimethyl-2,6-octadienyl)xanthone (6) and cowanin (7). Complete NMR assignment of 6-hydroxycalabaxanthone (2) and cowanin (7) were published for the first time. All isolated compounds were subjected to MTT assay against MCF-7, H-460 and DU-145 cancer cell lines. a-mangostin (5) and cowanin (7) were found to be potent against MCF-7 while 6-hydroxy-calabaxanthone (2) potent against DU-145 cell lines. Conclusion: The results indicate that G. cowa stem bark could be one of important sources of natural cytotoxic compounds.