The effect of ozone additive on the enhancement of the burning velocity for premixed methane-air flames is investigated by both experimental measurements and kinetic simulations. Laminar burning velocities with and without O-3 were directly measured using the Heat Flux method. The O-3 molecules were introduced into the system by a dielectric-barrier-discharge ozone generator installed in the O-2 gas line, which provided prompt control of on/off of the O-3 feed into the system, enabling a precise comparison of the measured burning velocity with and without ozone additives. Noticeable burning velocity enhancement was observed at off-stoichiometric conditions rather than stoichiometric conditions. With 3730 ppm O-3 additive in the oxidizer, experimental data shows similar to 8% burning velocity increase in fuel-rich mixtures and similar to 3.5% burning velocity increase for the stoichiometric mixture. With 7000 ppm ozone additive in the oxidizer, maximum similar to 16% burning velocity increase was observed at fuel-lean conditions while similar to 9.0% was found at fuel-rich conditions. An O-3 kinetic mechanism involving 16 elementary reactions together with the GRI-Mech 3.0 was composed and validated through CHEMKIN calculations, which gives good predictions of the burning velocities with and without O-3 additives. Extra O radicals contributed by O-3 molecules in the pre-heat zone initiate and accelerate the chain-branching reactions and consequently increase the burning velocity. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.