This paper reports the synthesis of ammonia from nitrogen and hydrogen as a model reaction in atmospheric microwave plasma. From the obtained results, we then investigate the effects of hydrogen addition to a plasma gas and hydrogen injection into the afterglow region of the plasma on ammonia production. The results showed that the addition of hydrogen to a plasma gas depressed nitrogen activation in the plasma. Hydrogen injection into the afterglow region of the plasma enhanced ammonia production. Dilution of a plasma gas with argon was effective for promoting the formation of active nitrogen species, resulting in the enhancement of ammonia production. The results suggest that active nitrogen species played a key role in ammonia production using atmospheric microwave plasma. (c) 2007 Elsevier B.V. All rights reserved.