Ammonia (NH3) is a potential hydrogen carrier as alternative fuel and feedstock for hydrogen production. In this study, plasma synthesis of NH3 was conducted in a packed bed dielectric barrier discharge (DBD) reactor using Al2O3 as the catalyst. In order to explore the mechanism of hydrogenation of plasma-excited nitrogen for NH3 synthesis, the whole NH3 synthesis process was divided into three steps including N-2 activation, hydrogenation of plasma-excited N(a), and desorption of NH3(a) from catalyst. The effects of reaction conditions on the three steps and corresponding NH3 production were examined. Results showed that more plasma-excited nitrogen species were formed through N-2 activation at higher N-2 flow rate, discharge time and discharge power for N-2 activation. Hydrogenation of plasma-excited N(a) to form NH3(a) was improved by more discharge time at the second step. Higher discharge temperature for N(a) hydrogenation favored NH3 (a) desorption from catalyst and increased NH3 production at the second step, with the total NH3 yield slightly changed. In addition, one-step NH3 synthesis in plasma was investigated and compared with the three-step process. The results will provide reference for catalyst and reactor design in plasma synthesis of ammonia. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.