The characteristic of active species formation in aqueous phase is investigated by applying a high voltage pulse to needle-plate electrode configuration. Radicals (. OH, . O, . H) were detected by analyzing emission spectra from the discharge channel. It is found that the radical formation is slightly delayed (similar to 1 mu s in the order of magnitude, depending on rising time of the applied pulse) from the peak of the applied voltage. Formation of radicals is also influenced by pulse wave shape such as half width and peak voltage, and oxygen gas bubbling into the discharge region. Hydrogen peroxide is formed by the pulsed discharge where the formation rate is larger in the case with oxygen gas bubbling than that of no bubbling.