The effect of 18 kinds of Chinese herbal medicine for the synthesis of TXA2 and PGI2 was studied. The porcine lung microsoma was used as donor of enzymes such as cyclo-oxygenase, thromboxane A2 synthetase and prostacyclin synthetase, etc. It was found that Glehnia littoralis could inhibit the synthesis of TXA2 and increase the formation of PGI2. Rheum palmatum (300 mg group) and Erigeron breviscapus significantly inhibited the synthesis of TXA2, but no apparent effection on the synthesis of PGI2. When Codonopsis pilosulae, Astragalus membracaceus, Angelica sinensis, Ginsenosides and Baicalin, etc. markedly inhibited the formation of TXA2 and mildly affected the formation of PGI2. Some Chinese herbs for promoting blood circulation play an important role on the inhibition of TXA2 synthesis. Some tonic herbs could either inhibit the synthesis of TXA2 and increase the synthesis of PGI2 or inhibit the formation of PGI2. Therefore these tonic herbs had the characteristics of both strengthening the body resistance and eliminating pathogenic factors. In this aspect they are better than the control drug (Aspirin) and other herbs of promoting blood circulation.