The dichloromethane extracts of the above ground parts of Hymenoxys brachyactis afforded three sesquiterpene lactones already reported, one new sesquiterpene lactone, biennin C and hispidulin as known toxic flavone. Structures of all compounds were established by spectroscopy and biennin C was determined as an adduct of the modified pseudoguanolide and hymenoxon by Gas Chromatograpy and MS spectrometer These sesquiterpene lactones have the same ${alpha},{eta}$-unsaturated functional group like that of hymenovin which has been known as major toxic constituent of important livestock poison. And biennin C is also considered as toxic compound because of toxic hymenoxon.