Abstract: A new thymol derivative, 8-hydroxy-9 , 10-dioxyisopropylidene- thymol (1 ) , together with three known thymol derivatives 10-hydroxy-8, 9-dioxyisopropylidene-thymol (2 ) , 8-hydroxy-9 , 10-diisobutyryloxy- thymol ( 3 ) and 8 , 10-dihydroxy-9-isobutyryloxy- thymol (4) , was isolated from 95% EtOH extract of the flowers of Inula helianthus-aquatica. Their structures were elucidated on the basis of detailed spectroscopic analysis . These compounds showed cytotoxicities on six cancer cell lines (K562 , HT-29 , SGC-7901, DU145, MDA-MB-231 , U251) , and compound 2 is the strongest one with IC50s of 4 .20 - 33. 12μmol􊄯L . This is the first time that the cytotoxic thymol derivatives were isolated from this titled plant .

Key words: Inula helianthus-aquatica