Manganese oxide was prepared by an irradiation of ultrasound to an acetone solution of potassium permanganate. The chemical analyses of gas, solution and products have shown that the synthetic reaction was not peculiar to sonochemistry but similar to the reaction in the basic aqueous solution. The product contained manganese oxide and by-products such as acetate and oxalate. Those by-products were difficult to be removed thoroughly and a part of them resided even after wash. From XRD, the obtained manganese oxide was likely to be α-MnO2. The product was examined as electrode material of lithium ion battery, and showed 180mAhg−1 of specific capacity under 0.5Ag−1 of current density. Moreover, it could be discharged and charged under a high level of current density (ca. 100mAhg−1 at 17C). The capacity deterioration caused by cycle repetition was also found to be much small.