The effects of eicosapentaenoic acid (EPA) on experimental thrombosis and its mechanism were studied. The results showed that eicosapentaenoic acid had a significant antithrombolic effect both in vivo and in vitro . EPA also had a fibrinolytic activity and can shorten euglobulin lysis time. Our studies also showed that EPA can inhibit platelet aggregation in rats. The plasma concentration of 6-keto-PGF1a and TXB2 was measured by radioimmunoassay. The results showed that EPA can reduce the contents of TXB2 and raise 6-keto-PGF1a/TXB2 ratio.