The conversion ratios of fuel-N to NO were investigated during combustion of two Chinese bituminous coals in four different atmospheres (O-2/N-2, O-2/Ar, O-2/CO2, and O-2/RFG) in an electrically heated drop tube furnace at elevated temperatures (1273-1673 K). The oxygen concentration in the inlet gas was changed from 8%. to 40%. The recycling ratio of flue gas ranged from 0.4 to 0.8. Through comparing the conversion ratios of fuel-N to NO among the four different atmospheres, the contributions of elimination of atmospheric nitrogen, high CO2 concentration in the furnace, and reduction of recycled-NOx to the reduction of total NOx in oxy-coal combustion were investigated. Results show that the contribution of recycled NO reduction was dominant It contributed 45-70% of the total reduction of NOx. It decreased with oxygen concentration in the inlet gas and increased with flue gas recycling ratio. The contribution of elimination of atmospheric nitrogen accounted for 12-25% and had a trend opposite that observed for the recycled-NO reduction. The contribution of the high CO2 concentration in the furnace accounted for 18-33%. It initially increased with oxygen concentration, and then decreased as the oxygen concentration reached and exceeded 21%. The influence of temperature on the contributions of elimination of atmospheric nitrogen, high CO2 concentration in the furnace, and reduction of recycled-NOx to the reduction of total NOx was complex. It depended on,combustion conditions and the type of employed coals.