The effects of NO and O 2 on the decomposition of NH 3 were studied at low concentrations (300–900 ppm) in N 2 under irradiation by electron beams of 1.5 MeV energy at 120°C. The NH 3 was decomposed to H 2 and N 2 in N 2, with a G-value of 1.3. In the presence of NO(800 ppm) or O 2 (5%), the NH 3 decomposition was much enhanced (G-values were 3.1 and 3.0, respectively), the H 2 formation was suppressed, and N 2O formation was observed with a high yield. The enhancement of the NH 3 decomposition in the presence of NO or O 2 is attributed to the reactions of H and NH 2 radicals with NO or O 2, leading to the formations of N 2 O, N 2, H 2O, etc. In the presence of NO, the reaction of NO + with NH 3 and that of NH 3 + with NO may also be important for the NH 3 decomposition.