Human Plasmin (Plm) is the active form of the zymogen plasminogen (Plg), a key serine protease that involves in fibrinolysis, and numerous other physiological processes, e.g., extracellular matrix degradation, cell migration and wound-healing. Plm inhibitors, currently applied as antifibrinolytic agents, are critical in reducing blood loss and transfusion during many major surgeries. Aprotinin targeting directly to active site of Plm was widely used as an efficient antifibnolytic drug before its discontinuation in 2008. Another two clinic drug tranexamic acid (TXA) and e-aminocarproic acid (EACA) can weakly attenuate Plm activity by blocking its binding sites to substrate (fibrin) or cell receptors while also causing non-ischemic seizures. Therefore, it is essential to develop highly potent and specific Plm inhibitors as antifibrinolysis drugs. Sunflower trypsin inhibitor-1 (SFTI), is a potent trypsin inhibitor (Ki=100 pM) isolated from sunflower seed. This 14 amino acid cyclic peptide is characterized by its exceptional high potency and stability, thus making it an ideal drug template for developing new Plm inhibitors. Here, we report the atomic-resolution binary x-ray crystal structures of microplasmin (µPlm, the catalytic domain of Plm) complexed with four SFTI-1 variants specifically inhibiting Plm in the nanomolar/picomolar range. The four structures reveal the importance of residues Tyr4 and Arg7/Lys7 of SFTI-1 variants in keeping its high inhibitory activity to Plm. This is the first reported binary structures of µPlm and cyclic peptide inhibitor and is critical for further developing safe and specific antifibnolytic drugs.