A known steroid, in addition to triterpenoids, anthraquinones, naphthalenes and a new anthraquinone glycoside, xanthopurpurin 3-O-beta-D-glucoside, were isolated from the roots of Rubia akane grown in Taiwan. Mollugin, a naphthohydroquinone, showed strong inhibition of arachidonic acid (AA)-induced and collagen-induced platelet aggregation. In contrast, 2-methyl-1,3,6-trihydroxyl-9,10-anthraquinone, xanthopurpurin 3-O-beta-D-glucoside, and xanthopurpurin showed mainly strong inhibition of collagen-induced platelet aggregation.