A sesquiterpene lactone, 1-O-acetyl-4 R,6 S-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 IC50 value of 22.1 μM which is more potent than the positive control, L-N6-(1-iminoethyl)lysine (IC50 = 33.7 μM). Structure of compound 1 was identified by 1D and 2D NMR experiments and by comparison with the reference standard.