A water plasma was generated by DC arc discharge with a hafnium cathode and a cupper anode. Arc fluctuation in water plasma was examined by arc image observation synchronized with voltage measurement. The main parameters that cause arc fluctuation was a feed rate of solution. The dynamic behavior of the arc as a restrike mode has faster fluctuation than the decomposition process in the water plasma. Axial and radial extent of the arc is increased with increasing in the feed rate. Therefore, effective energy of the arc is decreased with increasing in the feed rate. Decomposition rate of the treated material can be improved by controlling the arc fluctuation in the anode nozzle.