New sesquiterpenes tanacetolide A-C (1-3) were isolated from a Tanacetum sinaicum extract together with known compounds (4-10). The structures were elucidated on the basis of MS and NMR spectroscopic data. All the compounds were evaluated for the inhibition of inducible nitric oxide (NO) production in a mouse peritoneal macrophage system. Iso-seco-tanapartholide-3-O-methyl ether (4) produced potent inhibition of NO production (IC50 = 1.0 μM). At the protein expression level, 4 elicited concentration-dependant down-regulation of inducible nitric oxide synthase.