A series of magnetic iron-magnesium mixed oxide catalysts (Fe1-xMgxOz) were synthesized via a novel coprecipitation method with microwave thermal treatment, and their activity in NH3-SCR was tested on a quartz fixedbed reactor. Physical and chemical properties of the catalysts were characterized by X-ray diffraction (XRD), N2-adsorption-desorption, scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). Fe0.8Mg0.2Oz with excellent N2 selectivity and resistance to SO2 and H2O was validated as the proper SCR catalyst, with the maximum NOx conversion of 99.1% fulfilled at 325 °C. Activity was strongly influenced by the γ-Fe2O3 crystalline phase, and magnesium existed in an amorphous phase and interacted with iron oxide intensively to form solid solution in favor of SCR. For Fe0.8Mg0.2Oz catalyst, optimum pore diameter distribution, appropriate surface area, pore volume and abundant lattice oxygen on the surface could be guaranteed, which is good for the diffusion process and enhances the activity.