A protein designated as Flammulina Velutipes (FV) protein was isolated from the cultured mycelium of FV and purified to homogeneity by CM cellulose CM 52 and gel filtration chromatography. The molecular weight of FV protein were found to be 1.75?10~4u with pI 4.3. Amino acid analysis of FV protein indicated the presence of 14 kinds of amino acids, especially rich in Glu, Asp and Thr.Potent inhibitory effect of FV protein on thrombus formation, in the blood was observed in vitro when rats were given intravenously. The length of thrombus was shortened and the dry weight of the thrombus decreased significantly at the dose of 7.5?10~(-7) mol/kg. Thrombus formation was completely inhibited at the dose of 10~()-8 mol/kg. The IC_(50) was found to be 4.4?10~(-7) mol/kg. FV protein was shown to inhibit collegen induced aggregation of rat platelets both in vitro and in vivo. Its concentration for complete inhibition in vitro was 7.5?10~(-5) mol/L. The IC_(50) was found to be 4?10~(-5)mol/ L. However, FV protein dose of 1.25?10~(-5) mol/kg was required for 60% inhibition in vivo