This Paper evaluates the thrombolytic effect of freeze-dried Natto products(referred to as: freeze-dried natto) by dynamic measuring the change of blood pressure,the size of thrombus and the activity of plasminogen activator inhibitor substances(PAI) or tissue plasminogen activator(tPA) in the thrombus mode.S-D rats were divided into five groups randomly(N = 40),fed with different concentrations of freeze-dried natto or the same dose of Lumbrokinase respectively.The femoral artery pressures were traced by Powerlab /4s.The PAI and tPA activity were detected by ELISA.The femoral artery pressure of rats fed with freeze-dried natto,fed with Lumbrokinase and fed with normal chow were all at the same level(P > 0.05).The decrease level of femoral artery pressure of rats fed with freeze-dried natto or Lumbrokinase were lower than the rats fed with normal chow since after the thrombus forming for 40 min(P < 0.001).The wet weight or dry weight of the thrombus were significantly lower on the rats fed with freezedried natto than that on the rats fed with normal chow(P < 0.001).Compared with the rats fed with normal chow,the tPA activity increased as well as the PAI/tPA ratio decreased obviously in plasma of the rats fed with freeze-dried natto or Lumbrokinase(P < 0.001).The freeze-dried natto can inhibit the formation of arterial thrombosis.