Abstract: The folk drug of Fagopyrum cymosum (Trev.) Meisn has been reported to have pronounced therapeutic effect in the treatment of pulmonary sepsis. The present paper deals with the elucidation of the active principles of this plant. Three components were isolated. Component A was the main constituent in this drug and accounted for its therapeutic effect. The octamethylether, octamethylether diacetate and decanacetate derivatives were prepared from this component. On the basis of spectroscopic analyses, degradation products and physico-chemical constants, component A was identified as the dimer of 5, 7, 3', 4'-tetrahydroxyflavan-3-ol(C4-C8 linked), named dimeric procyanidin.Components B and C were identified as heeogenin and β-sitosterol, respectively.