NO removal activity and the durability of iron-exchanged mordenite type zeolite catalyst (FeHM) have been examined in a continuous fixed bed flow reactor. The catalytic activity for NO reduction by NH₃ in the presence of oxygen was much higher than that in the absence of oxygen, and it was fully reversible with respect to the presence of oxygen in the feed gas stream. The oxidation ability of SCR catalysts including FeHM was critical for both reactions of NH₃ and SO₂ oxidation, thus for the NO removal activity and its sulfur tolerance. The maximum conversion of NO for FeHM catalyst with respect to the reaction temperature shifted to the higher temperature due to its mild oxidation ability. The deactivation behaviors such as the changes of the physicochemical properties of the catalyst and the loss of NO removal activity induced by SO₂ could not be distinguished, regardless of the metals exchange in zeolite. However, the amount of deactivating agents deposited on the catalyst surface depended on the species of metals exchanged on the mordenite type zeolite, which was mainly attributed to the oxidation ability of metals for SO₂ conversion to SO₃.