Objective: To investigate the effect of leech injection on thrombosis and plasma tissue plasminogen activator (t PA) and its inhibitor (PAI) in the thrombosis model of mice. Methods: The different dose leech injection of different doses were given to mice in different groups for 7 days respectively. Thrombosis was induced in mice by carrageenan. The plasma t PA and PAI activity were detected by chromogennic substracte analysis.Results: The average thrombosis length of tail in the groups of leech injection were significantly shorter than the length in the groups of thrombosis control. Compared with the group of thrombosis control, the plasma PAI activity in the leech injection groups was lower. Conclusions: The leech injection can reduce thrombosis in mice. The mechanism might be associated with reducing the release of PAI.