Phytochemical study on the ethanol extract of a Crinum species by bioassay-guided fractionation resulted in the isolation of an active principle, which was determined as parthenin (1) on the basis of physicochemical and spectroscopic analyses. This is the first report on the existence of a sesquiterpenoid from Crinum ensifolium. Compound 1 was found to show strong cytotoxic activity against some cancer cell lines and strongly inhibit NF-${kappa}$B activity with the $IC_{50}$ value of 1.82 ${mu}M$.