One new unusual sesterterpenoid (1), four new sesquiterpene dimers (2–5) together with nine known sesquiterpenes (6–14) were isolated from the aerial parts of Inula britannica. The structures of the new compounds were elucidated by detailed spectroscopic analysis, including HR-ESIMS and 2D-NMR spectroscopic methods. In addition, compounds 1–8 and 10–11 were tested for their inhibitory effects against LPS-induced NO production in RAW264.7 macrophages.