The NO absorption characteristic of calcium hydroxide was investigated in a fixed bed reactor system in the presence of SO2. The effect of O-2 and H2O on NO removal was discussed in or without the presence of SO2. When SO2 was excluded from the flue gas, NO removal was slight. The presence Of SO2 in the flue gas Could enhance NO removal only when both of O-2 and H2O were also present. The essence of enhancement of SO2 oil NO removal was accelerating the conversion of NO to NO2. The desulfurization reaction product could not oxidize NO to NO2. The bed height experiment revealed that the multimolecular intermediate species, which Could decomposed in to NO2 was not formed from the adsorption of NO, H2O. and O-2 Oil the surface of absorbent. The possible reaction path proposed in paper is that there may be some Unstable intermediate species formed from the desulfurization reaction, which call enhance the conversion of NO to NO2. The presence of H2O can enhance the presence of unstable intermediate. According to analysis. the possible Unstable intermediate is the OH radical.