In this study, anti-mildew activities of the essential oils of the leaves, wood, twigs, flowers, and their constituents from Michelia comnpressa var. formosana were evaluated in vitro against 7 mildew fungi. The main compounds responsible for the anti-mildew activities were isolated and identified. The essential oils from the fresh leaves, wood, twigs, and flowers of M. compressa var. formosana were isolated using hydrodistillation in a Clevenger-type apparatus, and characterized by GC-FID and GC-MS. The essential oil yield from the flowers was the highest, and the oils obtained from the 4 parts of the tree were composed mainly of sesquiterpenoids. The leaf oil consisted primarily of α-cadinol (18.9%) and germacrene D (18.5%); the twig oil's main constituent was α-cadinol (19.1%); the wood oil's major components were α-cadinol (25.7%) and β-eudesmol (20.2%); and the flower oil contained α-cadinol (11.4%). Comparing the anti-mildew activities of the oils suggested that wood oil was the most effective. Further fractionation of the wood oil produced α-cadinol, β-eudesmol, τ-cadinol, and elemol. The 4 compounds exhibit very strong anti-mildew activities, and these in descending order are α-cadinol, elemol, β-eudesmol, and τ-cadinol. For the anti-mildew activities of the wood oil, the active source compounds were determined to be α-cadinol, elemol, β-eudesmol, and τ-cadinol.