The influence of catalyst electric conductivity on methane activation in a planar-type dielectric barrier discharge reactor is investigated by empirically comparing the degree of methane conversion of bare Al2O3 with that of Pt/Al2O3; from this, it is determined that the latter catalyst converts less methane owing to the presence of Pt. Calculations and comparisons of electric fields with and without Pt show that the presence of a Pt catalyst results in a lower electric field than does bare Al2O3. An analysis of product gases based on the correlation between the fragmentation of radicals and the electric field also indicates that the electric field is decreased by using Pt. From these results, it can be concluded that the synergies between the plasma and the conductive catalysts need to be reassessed for different electric field conditions, and that further studies of non-conductive catalysts that can enhance methane activation and synergistic effects are needed.