The on-board reforming technique for de-NOx is attracting increasing interest because of the fast start-up and size of the device. However, due to the limited amount of energy available from the battery, a full understanding of the characteristics of the plasma reforming reaction is important to optimise this technology to achieve hydrogen production with minimal power consumption. This paper reports the relative role of the thermal effect of plasma in octane oxidation induced by rotating arc plasma. The thermal effect or gas temperature increased with increased electric power; the conversion of the electric energy to heat was estimated to be higher than 40%. The increased thermal effect was the main consequence of further increasing the plasma power, which controlled the syngas yield and was the main determinant of the energy efficiency of the reaction. In comparison, although the O-2/C ratio was the main parameter determining the thermal environment in the reactor, it had only a small influence on the energy efficiency of the reaction. The optimal O-2/C ratio for maximum energy efficiency depends on the electric power (e.g. the optimal O-2/C was 0.8 at 80 W and 0.5 at 280 W). The results provide guidance for determining the optimal conditions for plasma-driven reforming processes.