Five neolignans (1 - 4, 8), two sesquiterpene-lignans (5 - 6), and two phenylpropanoids (7, 9) were isolated from the stem bark of Magnolia obovata Thunberg (Magnoliaceae) by repeated column chromatography. The structures of isolated compounds were identified as 4-methoxyhonokiol (1), obovatol (2), magnolol (3), honokiol (4), eudeshonokiol B (5), eudesobovatol B (6), coumaric acid (7), magnaldehyde B (8), and ${ ho}-coumaric$ acid (9) on the basis of spectroscopic analysis including 2D-NMR and MS data. Compounds 1 - 9 were evaluated for their anti-complement activities against the classical pathway of the complement system. Of them, compound 8 showed significant anti-complement activity on the classical pathway with $IC_{50}$ value of 102.7 ${mu}M$, whereas compounds 1 - 7 and 9 were inactive. This result indicated that an aldehyde group in the neolignan is important for the anti-complement activity against the classical pathway.