For the enhancement of ozone oxidation performance, a gas-liquid simultaneous injection nozzle was set at the upper end of the downcomer of bubble column. The flow characteristics and the decomposition performance were examined. Gas and liquid from the nozzle flowed downward in the downcomer and upward in the annulus between the column and the downcomer. A part of liquid in the annulus was entrained into the downcomer. Both the liquid circulation now rate and volumetric mass transfer coefficient increased with increasing downcomer diameter and with decreasing nozzle diameter. In the dye decomposition by means of air containing ozone, the decomposition conversion of dye was enhanced by the liquid entrained from the annulus to the downcomer at the upper end of the downcomer. On the basis of the balance equations of ozone and dye, the concentration distributions of ozone and dye, and the decomposition conversion of dye were calculated by using the flow characteristics.