A sesquiterpene lactone, 1-O-acetyl-4R,6S-britannilactone (1) isolated from the flowers of Inula britannica L. var. chinensis (Rupr.) Reg. (Compositae), was found as an iNOS inhibitory constituent for the first time with an $IC_{50}$ value of 22.1 $mu$ M which is more potent than the positive control, L-$N^6$-(1-iminoethyl)lysine ($IC_{50} =33.7 mu$ M). Structure of compound 1 was identified by 1D and 2D NMR experiments and by comparison with the reference standard.