In the course of our work on anti-platelet constituents from plants, five phenolic compounds, magnolol, honokiol, obovatol, methyl caffeate, and syringin, were isolated from the methanol extracts of the barks and fruits of Magnolia obovata. The compounds were identified based on the spectroscopic data. Methyl caffeate was isolated for the first time from the genus Magnolia and it showed 3∼4-folds higher potency than ASA. The activities of obovatol and honokiol were comparable to ASA. Magnolol and syringin showed only very mild inhibitory effects to all the stimulators.