The anticoagulant properties of materials derived from the seeds of Cassia obtusifolia were evaluated against platelet aggregation induced by arachidonic acid (AA), collagen, platelet activating factor (PAF), and thrombin, and these effects were then compared to those of three commercially available compounds. The active constituent of C. obtusifolia seeds was then isolated by silica gel column chromatography and HPLC, after which it was characterized as alizarin (1,2-dihydroxyanthraquinone) by spectroscopic analyses. With regard to 50% inhibitory concentration ($IC_{50}$) values, alizarin was found to effectively inhibit platelet aggregation induced by AA ($IC_{50}$, 53.1 M) and collagen ($IC_{50}$, 6.9 M), whereas the $IC_{50}$ values of its derivatives, alizarin-3-methylimino-diacetic acid were 46.3 M (AA) and 2.7 M (collagen). However, 1,4-dihydroxyanthraquinone and 1,8-dihydroxyanthraquinone exerted no anticoagulant activity against platelet aggregation. Moreover, alizarin and alizarin-3-methyliminodiacetic acid were found to be significantly more potent inhibitors against platelet aggregation induced by collagen than aspirin. Furthermore, the effect of alizarin and alizarin-3-methyliminodiacetic acid against platelet aggregation induced by AA was 1.5 and 1.3 times lower than that of aspirin. Taken together, these results indicate that alizarin and alizarin-3-methyliminodiacetic acid may prove useful for the inhibition of platelet aggregation induced by collagen.