In this study, we assessed the antitumor activity of the methanol extract from wood chips of the heartwood of the Taiwan red cypress, Chamaecyparis formosensis Matsumura, which is a precious tree species endemic to Taiwan. A brine shrimp lethality test (BST) indicated that the ethyl acetate (EtOAc)-soluble extract from the MeOH extract was a suitable candidate (LC (50) = 15.36 microg/mL) for further studies of the antitumor activity of its components. From this EtOAc fraction, we isolated six lignans and two norlignans and tested their cytotoxic activities IN VITRO against promyelocytic leukemia (HL-60) and hepatoma (Hepa-G2) cell lines. Among these compounds, 7,7'-( S)-dihydrotaiwanin C, isolated for the first time from nature, with its single crystal structure depicted in this study, exhibited significant cytotoxic activity against HL-60 cell lines IN VITRO (IC (50) = 4.03 microg/mL) after 24 hours. BST:brine shrimp lethality test Hepa-G2:human hepatoma HL-60:human leukemia IC (50):half-maximal inhibitory concentration SARs:structure-activity relationships AS:adenine sulfate.