The study was aimed to investigate the potential effect of procyanidins (PC) as a antithrombotic agent and its mechanism. 48 male SD rats were randomized into 6 groups, which include 8 rats each. Group A was normal control, group B was model control (no treatment), group C was group treated with aspirin [10mg/(kg x d)], groups D, E and F were treated with low, medial and high dose [100, 200 and 400 mg/(kg x d)] of PC respectively. In accordance with Kurz's protocols, rat's model of thrombosis of common carotid artery was contructed with FeCl(3), but for goup A 0.9% of normal saline was used for 20 minutes. The thromboxane B2 (TXB(2)), 6-Keto-PGF1alpha and GMP-140 contents in plasma were measured. The results showed that compared with the normal control group, the contents of TXB(2) and GMP-140 in plasma markedly increased in all of PC groups and aspirin group, and the contents of 6-Keto-PGF1alpha in plasma decreased. Compared with the model group, the contents of TXB(2) and GMP-140 in plasma markedly decreased in all of PC groups and aspirin group, and the contents of 6-Keto-PGF1alpha in plasma increased. Compared with the aspirin group, the contents of TXB(2) and GMP-140 in plasma reduced in all of PC groups and the contents of 6-Keto-PGF1alpha in plasma increased which was obvious in PC 400 mg/(kg x d) group. It is concluded that PC shows obvious anti-thrombosis effect, its mechanism closely correlates with inhibition of platelet activation and aggregation as well as protecting vasoendothelial cells. Antithrombosis of PC shows significant dose-dependence. The effect of the PC 400 mg/(kg x d) surpass the aspirin, but there is no significant difference between the PC 200 mg/(kg x d) and aspirin. This study provides experimental basis for clinical prevention and treatment of thrombosis.