To destruct tar which generated from a thermo-chemical conversion technology such as pyrolyzer and gasifier, a gliding arc plasma reformer was designed and investigated. The plasma technology can be applied to tar problems in downstream device of pyrolyzer or gasifier. An experiment was conducted using naphthalene as a surrogate tar, which is generated during the pyrolysis and gasification of waste or biomass, as the representative tar substance. To identify the characteristics of the influential parameters of tar decomposition, tests were performed on the steam feed rate, input tar concentration, total gas feed rate and SEI (specific energy input). The reference conditions of the gliding arc plasma reformer were 2.63 L/min in steam feed rate, 0.4 g/m3 in naphthalene concentration, 26.5 L/min in total gas feed rate, and 0.1 kWh/m3 in SEI. The naphthalene decomposition efficiency was 90.4%, and the energy efficiency was 3.4 g/kWh. Naphthalene was decomposed into light gases (H2, CO, and CO2), hydrocarbons (CH4, C2H4, and C2H6) and carbon black. Key Words : Gliding arc, Tar, Plasma reformer, Naphthalene, Pyrolysis.