Guided-mode resonances in subwavelength gratings are strongly dependent on incident angles. A beam selector with high sensitivity is designed based on this property. A guided-mode resonant silicon nitride (Si3N4) grating on a silicon dioxide (SiO2) substrate is presented as an example of such a device with a 1.55 μm wavelength. The calculated reflection resonance has an incident angle FWHM of Δθ~0.0042 radians, and a normal-incident peak efficiency of ~99.9%. In addition, we propose that this type of beam selector be applied to select transverse modes of semiconductor lasers.