Oxy-fuel combustion is one of the near-zero emission clean coal technologies which can realize large-scale CO2 capture, low NOx emission, and easy removal of SO2. This paper focuses on the reduction mechanism of NOx during oxy-fuel combustion. The influence of high concentration CO on the NO reduction with coal char, ash, and various metal materials has been studied in a laboratory-scale quartz reactor. Experimental results indicate that the presence of high concentration CO plays a crucial role in reducing NO on the coal char surfaces. Effect of NO concentration varies with different reaction conditions and different char types, which may be attributed to the different kinetic reaction orders. Moreover, both the reaction temperature and char prepared temperature have a pronounced effect on the char–NO–CO reaction. The active components in coal ash exhibit a clearly catalytic effect on the NO reduction with high concentration CO. Furthermore, the catalytic effect of various metal oxides, as well as coal ash, is significantly improved with the increase of temperature. It is found out that the catalytic activity in NO–CO reaction is Fe2O3 > CaO > MgO > Al2O3 > SiO2. Fixed carbon is more active on the NO reduction than coal ash. Kinetics analysis indicates that the apparent activation energy of the NO reduction with coal char is reduced by the presence of high level CO. In addition, large amounts of active metal oxides can also decrease the apparent activation energy of NO reduction reactions, which is beneficial for further reducing NO emission during oxy-fuel combustion.