Wasp is an important venomous animal that can induce human fatalities. Coagulopathy is a clinical symptom after massive wasp stings, but the reason leading to the envenomation manifestation is still not known. In this paper, a toxin protein is purified and characterized by Sephadex G-75 gel filtration, CM-Sephadex C-25 cationic exchange and fast protein liquid chromatography (FPLC) from the venom of the wasp, Vespa magnifica (Smith). This protein, named magnvesin, contains serine protease-like activity and inhibits blood coagulation. The cDNA encoding magnvesin is cloned from the venom sac cDNA library of the wasp. The deduced protein from the cDNA is composed of 305 amino acid residues. Magnvesin shares 52% identity with allergen serine protease from the wasp Polistes dominulus. Magnvesin exerted its anti-coagulant function by hydrolyzing coagulant factors TF, VII, VIII, IX and X.