Formation of bromate(BrO₃^-) in aqueous solution in the presence of Br^- under ultrasonic irradiation was investigated. The results show that BrO₃^- formed by a series of reactions between hydroxyl free radical (·OH) generated by ultrasonic cavitation and Br^-. The total BrO₃^- yield increases with the increase of the initial concentration of Br^-. While the yield ratio of BrO₃^- to Br^- decreases with the increased initial concentration of Br^-. The yield of BrO₃^- at different frequencies follows the order 400 > 600 > 800 > 200 kHz. When the ultrasonic energy density increases from 0.06 W/mL to 0.19 W/mL, the formation rate of BrO₃^- increases first and then decreases. BrO₃^- formation rate decreases with the increase of pH.