A novel decomposition treatment for halogenized compounds using a carbon nanotube (CNT) electron source has been proposed. It is observed that a high concentration of chlorophenol can be significantly decreased by using a CNT electron source. The concentration is reduced to less than 1/1000 after only a few minutes of treatment. The input energy for 1 g of chlorophenol is 46 J when the injection power is 0.5 W. The input energy is only 1/161 times lesser than that for the treatment for barrier discharge. The proposed treatment using CNTs has a high efficiency because the input energy is provided only by the accelerated electrons.