This report deals with the koji protease activity and cell numbers of a mixed koji-making system by Aspergillus niger and Aspergillus oryzae on different conditions. It studies on the effect of temperature, time and mold ratio on koji . The result shows: temperature of 30℃, time of 35h , mold ratio ( Aspergillus oryzae to Aspergillus niger ) of 3∶1 are the optimum conditions. On this condition , the koji cell number is 8.66×109 cfu/g,protease activity is 2588U/g.