In order to enhance the biodegradability of the excess sludge generated from the wastewater treatment, a centrifugal vibration mill was used to mill the activated sludge. The centrifugal vibration mill had a column with a volume of about 10 x 10(-3) m(3) and the sphere stainless steel balls were used. The operational conditions were changed, and the biodegradability of milled sludge was evaluated by using the BOD. The ball diameter of 7.9 mm was optimum in this experimental condition. The biodegradability of sludge increased with the milling time and became constant beyond 12 minutes. It had a maximum when the effective volume fraction (the ratio of the sludge volume to the void volume in ball bed) was unity.