Genistein, one of the isoflavones in doenjang, is generally known to prevent various cancers, osteoporosis, climacterium, and menopause symptoms, and has better bioavailability and healthful physiological effects than its glucoside, genistin. In both traditional and commercial doenjangs, genistein content ranged from 370 to 1,510 mg/kg, however, significant amounts of genistin also existed at the level of 190 to 350 mg/kg. After treating with corn $eta$-glucosidase, over 84% of genistin in doenjang was converted to genistein. However, physiochemical characteristics such as pH, viscosity, 2-thiobarbituric acid (TBA) value, and color were not changed significantly after corn $eta$-glucosidase treatments. Therefore, this study shows that the improved doenjang with the increased genistein content can be produced using corn $eta$-glucosidase.