The effects of NOx and O, H and N radical additions on the extinction limits of H2/air and CH4/air mixtures were numerically investigated with detailed chemistry. The results showed that similar to the ignition limit, the extinction limits of both fuels were extended by the addition of radicals, though the effect on the extinction limit was much smaller than that on the ignition limit. In the case of low inlet temperature, where radicals initially added to a mixture were quickly quenched via recombination reactions, the combustion enhancement effect was mainly due to a thermal effect by heat release via recombination of radicals. The effect of radical addition on the extinction limit appeared more strongly when the inlet temperature was considerably increased. The addition of radicals to the H 2/air mixture at high inlet temperature caused a change of the extinction curve from the S-shaped curve to a monotonic single-valued curve. However, such a change in the flame curve by the addition of radicals did not occur for the CH4/air mixture, even at a much higher inlet temperature. On the other hand, it was shown that the addition of NO x, which is more stable than other active radicals at low temperatures, significantly extended the extinction limit of a lean CH 4/air mixture at high pressures.