Abstract: The oxidation of potassium iodide has been investigated under the irradiation of ultrasound and visible light respectively and simultaneously. We have observed that the amount of iodine liberated under the combined irradiation of ultrasound and visible light is greater than the sum of that under irradiation of ultrasound and visible light respectively, indicating a synergistic effect of ultrasound and visible light irradiations. Basing on the investigation of the reaction kinetics of iodine liberated, we have ascribed the synergistic effect to the perfect stirring of the photochemical reactor induced by the applying simultaneous ultrasound. This ideal stirring can result in the homogenization of the primary light effect in the whole reaction medium, which results in the acceleration of the photochemical reaction.