The carbon dioxide reforming of methane to synthesis gas was investigated in a dielectric barrier discharge reactor at room temperature. The influence of dilution of reactants by helium was studied. We showed that, at a fixed contact time, the conversions of CH(4) and CO(2) increase when the amount of helium in the gas mixture increases. This result is attributed to the "penning ionization" phenomenon, which corresponds to an energy transfer from excited He to molecules in ground state (CH(4), CO(2)). The selectivity to products is affected by the dilution factor. As soon as helium is present in a large amount the formation of products resulting from recombination of methyl radicals (such as C(2), C(3) and C(4)) is less favourable due to the lowest probability of collisions to proceed. A kinetic model is proposed based on the assumption that the reactant molecules CH(4) or CO(2) are attacked by active species produced by the plasma discharges, and the production of this active species are function of the plasma power. This model which takes into account the dilution by helium fits particularly well the experimental data we obtained.