The methane-air flamelet was studied with the effect of thermal radiation considered in this paper. The chemical reaction mechanism used is the latest detailed mechanism of GRI-Mech 2.11 which contains 49 species and 279 elementary reactions including C-2 chemistry of methane oxidation and chemistry of thermal NO and prompt NO. The numerical results show that flame radiation can induce another extinction limit at a low scalar dissipation rate (0.029 s(-1)), which is in addition to the well-known extinction limit caused by the overstretching of flame at a high scalar dissipation rate (18.4 s(-1)). The influence of thermal radiation on flamelet temperature and NOx concentration is found to be significant enough in the small to moderate range of the scalar dissipation rate to change the trend of the temperature and species mass fraction profiles in the direction opposite to that when the thermal radiation is ignored. NOx production is shown to be strongly dependent upon the flamelet temperature. In the methane-air flamelet structure, it is found that the major species, H2O, can be well predicted by the equilibrium state relationship for the scalar dissipation rates between the two extinction limits, but not CO2 or CO since they are affected not only by the mixture fraction, but also by the scalar dissipation rate. Also shown is that preheating the air side has a more profound effect on flamelet temperature than preheating the fuel side, and that raising either air or fuel temperature increases NOx and CO production. Finally, the effect of nonunity Lewis numbers of hydrogen (H-2 and H) on the flamelet structure is examined and shown to increase CO2, but decrease the flamelet temperature and mass fractions of H2O, CO, and NOx. The Lewis number is also found to have an important effect on the extinction limits of the scalar dissipation rate for methane-air flames.