A novel human antithrombin III (AT III) mutant having a high antithrombin activity in the absence of heparin and effective in the treatment of thrombotic disorders as an anticoagulant, which is obtained by mutating amino acids at the reactive site and the heparin binding site of human AT III into another amino acids with the use of the recombinant DNA technology with the use of a DNA coding for AT III as a template. The invention also relates to a method for mass producing the above-described mutant by incubating a host transformed by an expression vector having the cDNA of the mutant inserted therein.