A dielectric barrier discharge (DBD) plasma reactor for upgrading of anisole, a model compound representative of lignin-derived bio-oils, was investigated with helium as a carrier gas. The effects of carrier gas flow rate, liquid anisole feed flow rate, and reactor length on the reactor performance were investigated. As a result of the decomposition of anisole, the most prevalent free radical species that formed is inferred to have been phenoxy, resulting from the breaking of the C-methyl-O bond. The residence times of reactive species and feed molecules are inferred to be key parameters affecting the conversion of anisole as well as the distribution of products. The three main classes of reaction of anisole were demethylation to give phenol, transalkylation, yielding 4-methylanisole and methylphenols, and hydrogenolysis of phenol to give benzene. The optimal experimental conditions were the carrier gas flow rate of 100 mL/min, the feed flow rate of 0.1 mL/min, and the outer electrode length of 20 cm; the anisole conversion and input power under these conditions were 72.7% and 71.2 W, respectively.