In order to enhance the ammonia to hydrogen (ATH) energy efficiency, systematic study was carried out with atmospheric pressure alternating current arc discharge reactor (using a pair of stainless steel (SS) tube electrodes). Results showed that, using small-diameter SS tube electrodes with small discharge gaps forced more ammonia molecules to go through the effective plasma volume and obtained high electrodes temperature. Adopting low discharge frequency increased the discharge time, the effective plasma volume and the electrode temperature. These changes can enhance both the gas-phase plasma decomposition and the electrode-surface catalytic decomposition of ammonia. Insulating the reactor significantly increased the electrode temperature in small-diameter reactor, so as to enhance the electrode-surface catalytic decomposition of ammonia. In large-diameter reactors, however, the electrodes temperature increased less rapidly and more ammonia bypass of the effective plasma volume occurred. A 12.5 mol/kW h ATH energy efficiency was reached when ammonia was completely converted under the conditions of electrode diameter 3 mm, electrode gap 4 mm, discharge frequency 5 kHz, reactor diameter 8 mm, NH3 flow rate 150 ml/min and input power 48 W. The ATH energy efficiency was further enhanced under similar conditions when incomplete ammonia conversion was allowed. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.