Strong nonreciprocal radiation effect, which is achieved by inserted a magneto-optical film between a one-dimensional metallic grating and a metallic reflector, is investigated. Strong nonreciprocal radiation is obtained at wavelength around 15.5 μm when the incident angle is only 12° and the external magnetic field is 2 T. Such phenomenon is attributed to the critical coupling between the metal grating and guided mode resonances (GMRs) excited in the magneto-optical film. The physical mechanism is revealed by investigating the magnetic field distributions and verified through the coupled mode theory (CMT). In addition, the performances of thermal radiation for the designed emitter remain well in a wide range of geometric dimensions, which is friendly for fabrication. This work provides a new approach for designing nonreciprocal thermal emitters working at small angles. We believe that the designed structure can be employed to demonstrate the Kirchhoff’s law with nonreciprocal materials and enhancing the efficiency of time-asymmetrical photovoltaics.