In the course of our work on anti-platelet constituents from plants, eight phenolic compounds, $({pm})-syringaresinol$ (1), 4-hydroxybenzaldehyde (2), 4-hydroxybenzoic acid (3), vanillic acid (4), 4-hydroxy cinnamic acid (5), quercetin 3-O-rhamnoside (6), rutin (7), and quercetin $3-(2^G-rhamnosylrutinoside)$ (8) were isolated from the methanol extract of the leaves of Magnolia obovata. The compounds were identified based on the spectroscopic data. Compound 2, 3, 5, 6, 7 and 8 were isolated for the first time from genus Magnolia. 1 and 6 showed same order of inhibitory potencies as acetylsalicylic acid (ASA) to rat platelet aggregation induced by all the stimulators tested. The remaining six compounds showed only mild effects.