Abstract: Ammonia cofiring ratio (R_NH3) is an important parameter affecting the carbon dioxide (CO₂) reduction of coal-fired units. Although high R_NH3 is beneficial to CO₂ reduction, it may lead to significant increase in nitrogen oxide (NO_x) emissions, which is going to hinder the application of ammonia/coal cofiring technology. In order to investigate the influence mechanism of R_NH3 on NO emission, a chemical reactor network model of one-dimensional furnace is developed to investigate the influences of R_NH3 on the NO production and reduction reaction pathways of NH₃ by means of detailed chemical kinetic model. The predicted results replicate the experimentally observed trend of NO emission that first increases and then decreases with the increase of R_NH3. It is shown that, under lower R_NH3 the concentrations of HNO/N increase with the increase of R_NH3 which leads to increased NO emission. Under higher R_NH3, however, most of the H/OH radicals are consumed by the dehydrogenation reactions of NH_3, which consequently inhibits the NO production reactions between the HNO/N and H/OH radicals, thus leading to reduced NO emission. Therefore, the root mechanism of the increase-then-decrease trend of NO emission with the increase of R_NH3 is attributed to the competition to the H/OH radicals between the initial and final steps of the NO production reaction pathway.