The characteristics of NO_x emission in the high-temperature air combustion technology were investigated numerically. Two kinds of methods were used. Detailed chemistry and transport properties were used for a two-dimensional laminar diffusion flame, and a simplified reaction scheme was used for calculating a turbulent flame in a furnace. Results show that the thermal mechanism dominates NO_x emission in high-temperature air combustion. If only the preheated air temperature is increased, NO_x emission will rise significantly. However, the combination of air preheating and flue gas recirculation not only improves the combustion efficiency, but also suppresses NO_x emission in the combustion process.