moderate or intense low-oxygen dilution (MILD) combustion a newly developing and innovative combustion technology. A MILD combustion without preheated air was achieved with inner flow recirculation and simulated by a computational fluid dynamics (CFD) model with GRI-Mech 3.0. Comparisons between the experimental and predicted results, including temperature and O-2, CO, and NO concentration fields, demonstrate that the model closely predicted MILD combustion without preheated air and is suitable for studying the NO formation mechanism under these conditions. According to the distribution of NO and NO2, there are three zones in the furnace: a central zone (I), a transition zone (II), and a recirculation zone (III). The NO concentration is lowest in the central zone, highest in the recirculation zone, and intermediate in the transition zone. The NO2 concentration is high in regions where the NO concentration is low and low in regions where the NO concentration is high. The NO formation mechanism is that NH3, N, and HCN are mainly produced in the recirculation zone, and the majority of NH3, N, and HCN are recycled into the central and transition zones. In the central zone, the paths (NH3 -> NH2 -> HNO -> NO), (N -> NO), and (NNH -> NH -> HNO -> NO) are main paths for NO formation and the majority of NO is oxidized to NO2 by reaction R186. The majority of NO2 in the central zone is transported to the transition zone via diffusion and convection. In the transition zone, the majority of NO2 is reduced to NO and the main resource of NO formation. Unreacted NO2 in the transition zone is transported to the recirculation zone and reduced to NO. Moreover, NNH is the main nitrogen-containing radical produced from N-2 by the reverse direction reaction R209 (NNH + H <-> H-2 + N-2) in the recirculation zone. NNH formed in the recirculation zone is recycled into the central and transition zones and reduced to N-2 by reaction R204 (NNH <-> N-2 + H).