Abstract: The effect of water vapor on NO reduction by methane over iron was investigated at 300~1 100 ℃ in an electrically heated ceramic tubular flow reactor in both N₂ and simulated flue gas atmospheres. The iron samples before and after reaction were characterized by XRD, SEM and XPS. The results demonstrated that water vapor has a small effect on NO reduction by methane over iron. In N₂ atmosphere, water vapor is involved in the oxidation of iron; compared with that in the absence of water vapor, the NO reduction efficiency is decreased slightly when 2.5%~7% water vapor is added into the reaction stream. However, the NO reduction efficiency increases with the increase of water content from 2.5% to 7%, as water vapor may promote the oxidation of iron, forming porous iron surface. Methane is involved in the reduction of the iron oxides, leading to the formation of a dense layer of Fe₃O₄ and FeO, which may inhabit the interaction of NO with metallic iron and then decrease the NO reduction efficiency, as compared with that without methane. In the simulated flue gas atmosphere, water vapor promotes the NO reduction by methane over iron. When the excess air ratio is 0.7 in reaction zone (SR₁) and 1.2 in burnout zone (SR₂), the NO reduction efficiency at 1 050 ℃ is 96.7% in the presence of 7% water vapor, compared with the value of 90.6% in the absence of water vapor. SO₂ causes a slight decrease of NO reduction. Long term test results showed that over iron at 1 050 ℃ in the simulated flue gas atmosphere containing 7% H₂O and 0.02% SO₂, NO reduction efficiency remains higher than 90% after reaction for 50 h in the presence of 1.14% methane.