The effect of the impregnation solvent in a Fe-ZSM-S system on the performance was evaluated for the selective catalytic reduction by ammonia (NH3-SCR) of nitric oxide (NO). As a result, water (H2O) as the solvent caused more iron clusters to form on the zeolite support and decreased the activity at low temperature. When absolute methanol was used, the iron particle was silk-like and uniformly distributed outside the pore structure of the zeolite, resulting in a poor performance at high temperature and poor sulfur dioxide (SO2) and H2O resistance. Also, for Fe-ZSM-5 impregnated with a mixed solvent, the iron particle was in excellent dispersion inside and outside the porous structure of the zeolite. A widened active temperature window, good SO2 and H2O resistance, and good hydrothermal stability were due to the appropriate ratio of Fe3+ and plenty of surface-adsorbed oxygen.