A cylindrical wetted-wall reactor in which high-voltage corona discharge was generated in gaseous space was applied to decompose phenol dissolved in water. It was found that the decomposition efficiency depends on current and current-density. When the current density was decreased by extending the axial corona region length at a constant current, electron efficiency decreased with the increase of corona region length in the range of relatively low discharge current, while this trend was reversed in the range of high discharge current. Experiments with varied applied voltages by the use of varied cathode diameters suggested that these reversed trends can be explained as follows. (1) The effect of increasing voltage at increasing current density, which increases the reactive radicals, becomes significant at the relatively low current condition. (2) At the high current condition, the voltage becomes excessively high so that ion-wind will disturb the uniformity of the discharge due to a waving water surface. Nevertheless, the condition with relatively low voltage by the use of long discharge region length and thin cathode diameter leads to high energetic efficiency in all discharge current ranges examined here.