The nitrogen oxides (NOx) formation characteristics of a one-dimensional premixed CH4/air flame using detailed-kinetic chemistry were examined numerically. The combustor length and the amount of heat loss were varied to investigate the effects of the heat loss and the residence time on NOx formation in the postflame region of the combustor. The following conclusions were drawn. Under the adiabatic wall condition, in the flame region, NO is primarily produced by the prompt NO mechanism; however, the thermal NO mechanism is more important than other mechanisms in the postflame region. Thus, an increase in the combustor length leads to an increase of residence time and causes a remarkable increase of the NOx emission index. Under the heat loss conditions in the postflame region, NO production does not occur additionally at the postflame region, and the NO level decreases considerably compared to that under adiabatic condition as the heat loss rate increases. Specifically, in the case of phi = 1.0, even when the temperature decreases by approximately 15% compared to that of the adiabatic condition in a combustor exit of 10 cm, the NO concentration is reduced significantly to 70%. From these results, it can be concluded that control of both the residence time and the heat loss in a practical combustor should be recognized as important factors for NOx reductions.