The denitration performances of transition metal oxide oxygen carriers (OCs) including CuO/Al2O3, MnO2/ Al2O3 and NiO/Al2O3 for the gas-fired boiler flue gas were investigated. The results showed that NO was able to be reduced to N-2 through a direct reaction with the reduced OCs, and at the same time, the reduced OCs were converted to the oxidized OCs. Then, the oxidized OCs were able to be regenerated to the reduced OCs by introducing reducing gases. As such, the continuous denitration was achieved based on a chemical-looping combustion (CLC) denitration mechanism. Among the reduced CuO/Al2O3, MnO2/Al(2)O(3)and NiO/Al2O3, the reduced CuO/Al2O3 showed the best denitration activity. The NO denitration based on the reduced CuO/Al2O3 and CO also included the catalytic mechanism, whereas the CLC mechanism was dominant. The denitration performance of CO as a reducing gas was significantly better than that of C2H5OH, CH3OH, H-2 or NH3. 8% CO2 in the flue gas inhibited the denitration below 250 degrees C, whereas 15% H2O in the flue gas improved the denitration above 125 degrees C. O-2 in the flue gas deteriorated the denitration through reducing the proportion of the reduced species in the OC, whereas a high NO conversion could be obtained through increasing CO concentration to make. less than 1.0. For the denitration of a simulating flue gas based on the reduced CuO/Al2O3 and CO, the temperature needed for a NO conversion of 90% was as low as 125 degrees C at 10,000 h(-1).