Abstract: The hydroxyl free radical, ·OH, was produced in Fricke solution under ultrasonic radiation with low frequency, and Fe2+ was oxidized to Fe3+ by the hydroxyl free radical in the solution. The output of hydroxyl free radicals in Fricke solution was determined indirectly by measuring the change of absorbance (ΔA) at 235 nm by UV-Vis spectrophotometry. Influential factors on the output of hydroxyl free radicals, including time of ultrasonic radiation, temperature, frequency and mode of ultrasonic treatment, were preliminarily studied.