Phytochemical investigations of Tsoongiodendron odorum and Manglietiastrum sinicum, both Magnoliaceae, led to the isolation of twenty compounds in total. Among them, one was a new sesquiterpene, 11-O-oleoyl-beta-eudesmol (2), and another, 1-(3,4-dimethoxypheny)-4-(3,4-methylenedioxyphenyl)-2,3-dimethy lbutane (12) was isolated as a natural product for the first time. Moreover, 13C-NMR spectral data of isoguaiacin (16) are reported here for the first time. Structure elucidations for compounds reported here were mainly based on their spectral data. The ethanolic extracts of T. odorum and M. sinicum, and six pure compounds, 4(15)-eudesmen-11-ol (beta-eudesmol) (1), 1 beta-hydroxy-4(15),11(13)-eudesmadien-12,6 alpha-olide (reynosin) (3), 3,11(13)-eudesmadien-12,6 alpha-olide (alpha-cyclocostunolide) (5), erythro-1-(4-hydroxy-3-methoxyphenyl)-4-(3,4-methylenedioxyphenyl)-2,3- dimethylbutane (11), nectandrin-B (18), and syringaresinol (19), displayed considerable inhibition against platelet aggregation induced by AA, by ADP, or by PAF.