Four flavonoids, dihydroartomunoxanthone (1), artomunoisoxanthone (2), cyclocomunomethonol (3) and artomunoflavanone (4), together with three known compounds, artochamins B (5), D and artocommunol CC (6) were isolated from the cortex of the roots of Artocarpus communis. The structures of 1-4 were determined by spectroscopic methods. The antiplatelet effects of the flavonoids, 1-3, 5 and 6 on human platelet-rich plasma (PRP) were evaluated. Of the compounds tested in human PRP, compounds 1, 5 and 6 showed significant inhibition of secondary aggregation induced by adrenaline. It is concluded that the antiplatelet effect of 1, 5 and 6 is mainly owing to an inhibitory effect on thromboxane formation.