Abstract NH3 is an important N-containing intermediate in solid fuel combustion, an efficient NO reducing agent and a carbon-free fuel. The transformation of NH3 is fairly complex in furnace and dominates the final NOx emission level. In this work, the effect of the CO/O2/CO2 system on NH3 transformation was researched in a self-designed flow reactor in a wide temperature range, 1073-1773 K. A small amount of NO net formation was detected in both the CO/NH3 and CO2/NH3 systems, while the mechanisms are different. The reaction between CO and NH3 is complex at low and intermediate temperatures, while the effect of CO2 is limited in the elevated temperature range. Significant amounts of NO forms in the O2/NH3 system, but the net formation of NO depends on the competition between NH3 oxidation by O2 and NO reduction by NH3. With CO oxidation included, the transformation of NH3 is dramatically enhanced. However, due to the rapid chemical reaction in the presence of O2 or CO oxidation, the mixing process in the front end of the reaction tube strongly influence the reaction results, leading to a fairly low conversion ratio of NH3 to NO at elevated temperatures.