Human　coagulation　factor　XII,　the　initiating　factor　in　the　intrinsic　coagulation　pathway,　is　critical　for　pathological　thrombosis　but　not　for　hemostasis.　Pharmacologic　inhibition　of　factor　XII　is　an　attractive　alternative　in　providing　protection　from　pathologic　thrombus　formation　while　minimizing　hemorrhagic　risk.　Large　quantity　of　recombinant　active　factor　XII　is　required　for　screening　inhibitors　and　further　research.　In　the　present　study,　we　designed　and　expressed　the　recombinant　serine　protease　domain　of　factor　XII　in　Pichia　pastoris　strain　X-33,　which　is　a　eukaryotic　expression　model　organism　with　low　cost.　The　purification　protocol　was　simplified　and　the　protein　yield　was　high　(~20　mg/L　medium).　The　purified　serine　protease　domain　of　factor　XII　behaved　homogeneously　as　a　monomer,　exhibited　comparable　activity　with　the　human　βFXIIa,　and　accelerated　clot　formation　in　human　plasma.　This　study　provides　the　groundwork　for　factor　XII　inhibitors　screening　and　further　research.　©　2019　Japan　Society　for　Bioscience,　Biotechnology,　and　Agrochemistry.