The selective catalytic reduction of NOx by ammonia (NH3-SCR) has widely applied in the abatement of NOx. Fe-ZSM-5 is a potential catalyst, showing high performance in standard SCR (mainly NO) at > 400°C and in fast SCR (NO/NO2 = 1) at ≥ 250°C. The identification of the active species in Fe-ZSM-5 catalysts and reaction mechanism for SCR reactions were investigated by in situ and operando EPR, UV-Vis and FTIR spectroscopy. The spectroscopic studies demonstrate the role of NO2 to reoxidize isolated Fe(II) to the active Fe(III) at β and γ sites, and attribute them the higher activity in fast SCR.