Anticoagulation factor Ⅰ (ACF Ⅰ) from the venom of Agkistrodon acutus prolonged plasma prothrombin time (PPT) with dose-dependent manner and exhibited marked anticoagulant activity only at the concentration higher than its critical concentration (12 nmol/L). It was discovered that ACF Ⅰ formed a 1:1 complex with activated coagulation factor (FXa) in the presence of Ca2+ ions by the method of polyacrylamide gel electrophoresis. Both native ACF I and decalcified ACF Ⅰ failed to form complexes with FXa in the absence of Ca2+. Sr2+ ions were able to replace Ca2+ ions in the binding of ACF I to FXa, but both Ba2+ ions and Tb3+ ions were ineffective. ACF Ⅰ was a new member of the IX/X-bp family in the C-type lectin superfamily, and had a amino acid composition similar to the other members of this family. It was composed of 251 amino acid residues with a molecular weight of 29 603.6 u on non-reducing condition, determined by MALDI-TOF-MS, and a molecular weight of 14.7 ku on reducing condition, determined Anticoagulation factor I (ACF I) from the venom of Agkistrodon acutus prolonged plasma prothrombin time (PPT) with dose-dependent manner and demonstrated stain anticoagulant activity only at the concentration higher than its critical concentration (12 nmol / L). It was discovered that ACF I formed a 1: 1 complex with activated coagulation factor (FXa) in the presence of Ca 2+ ions by the method of polyacrylamide gel electrophoresis. Both native ACF I and decalcified ACF I failed to form complexes with FXa in the absence of Ca 2+. Sr 2+ ions were able to replace Ca2 + ions in the binding of ACF I to FXa, but both Ba2 + ions and Tb3 + ions were ineffective. ACF I was a new member of the IX / X-bp family in the C-type lectin superfamily, and had It was composed of 251 amino acid residues with a molecular weight of 29 603.6 u on non-reducing condition, determined by MALDI-TOF-MS, and a molecular weight of 14.7 ku on reducin g condition, determined