NO reduction experiment was performed by injecting ammonia radicals, which were externally generated by flowing the NH3 gas diluted with Ar gas through a dielectric barrier discharge with a one-cycle sinusoidal-wave power source. The discharge was intermittently formed between coaxial cylindrical electrodes with a space of 1.5 mm at an applied peak-to-peak voltage of 3-25 kV The generated radicals were injected into simulation gas (NO/O-2/N-2). The simulation gas contained 0-5.6% O-2, and the effect of O-2 on NOx removal was discussed. The minimum reaction temperature for NO reduction was low when simulation gas contained O-2. High O-2 concentration (O-2 = 5.6%) in simulation gas, high repetition rate to NH3 and high applied power to NH3 decreased NO removal efficiency. (C) 2003 Elsevier B.V. All rights reserved.