We proposed a novel ammonia synthesis process in which the pressure of a N 2-H 2 nonthermal plasma system swung between low and high values to improve energy e ciency of the ammonia synthesis. We compared the energy eciency, calculated by the amount of ammonia produced and plasma input power between constant-pressure system and pressure-swing one. In the constant pressure system, the highest ammonia formation rate and energy e ciency were obtained at the lowest pressure. On the other hand, the pressure swing system provided a higher ammonia formation rate and energy e ciency than the constant pressure system even though the pressure-swing range is within that of the constant pressure system. Consequently, the highest ammonia formation rate and energy e ciency were obtained by the pressure-swing system. Additionally, the dependence of ammonia formation e ciency on the plasma input power was investigated. In the constant pressure system, the amount of ammonia produced increased linearly with plasma input power. Interestingly, however, in the pressure swing system, the amount of ammonia produced saturated with increasing plasma input power. Therefore, the amount of ammonia produced did not decrease signi cantly even when the plasma input power was reduced, and the highest energy e ciency of 2.0 g-NH 3 /kWh was obtained under the minimum plasma input power condition tested for the pressure swing system.