This work presents the effects of the insulation of ground electrode and operating parameters on CO2 reforming of CH4 to syngas in a coaxial-cylindrical dielectric barrier discharge (DBD) plasma reactor coupled with Ni/alpha-Al2O3 catalyst. For the conventional plasma reactor, abnormal outside discharge inevitably ignites and develops from the ground electrode, giving rise to the formation of harmful substances (e.g., NOx) and the waste of energy. The power dissipation for the conventional reactor therefore includes both that used for the dry reforming reactions and the loss of energy due to the air discharge. The new finding of this work is that by covering the ground electrode with an insulating oil jacket, not only the NOx formation is prevented but also the conversion rates, product selectivity and energy efficiency are largely enhanced by roughly 30, 10 and 100% at a specific energy input of about 47 kJ, respectively. The results are associated with the extinguishment of the discharge occurring outside the reactor, which is usually neglected when designing DBD reactors. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.