Falcarindiol was isolated as an algicidal principle against the harmful red tide dinoflagellate, Heterocapsa circularisquama, from Notopterygii Rhizoma through bioassay-guided separation.To determine the ambiguous absolute structure of this active principle, all three stereoisomers as well as falcarindiol were synthesized.  As a result of intensive analytical of their physicochemical properties, the configuration of Falcarinfiol was revealed to be 3R,8S.  (3S,8S)- and (3S,8R)-isomers were found to exhibit more potent algicidal activity than (3R,8S)-Falcarindiol isolated from Notopterygii Rhizoma.