Four new eremophilane-type sesquiterpenes, petasinones A (1), B (2), C (3), and D (4), together with petasin (5), isopetasin (6), petasol (7), isopetasol (8), S-petasin (9), and S-isopetasin (10) were isolated from the aerial parts of Petasites formosanus. The structures of new compounds were determined by chemical and spectroscopic methods.