The effect of low frequency airborne ultrasound energy on microbiological and physicochemical properties of soybean koji during preparation was investigated. Following inoculation of steam-heated soybeans with spores of Aspergillus oryzae, koji samples were treated with airborne sonication at increasing frequencies of 20, 25, 30, and 35 kHz and incubated at 30 o C for 5 days. Amylase and protease activities, the number of fungal spores, bacterial growth, the moisture content, pH, and the titratable acidity were analyzed during koji preparation. All frequencies of low ultrasound treatment tested enhanced enzyme activities. After continuous ultrasonication at 30 kHz, amylase, and protease activities were significantly (p<0.05) increased by 183 and 205%, respectively, relative to corresponding control groups.