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We numerically investigate innovative comb-like filters working in the telecommunication region based on guided-mode resonances in thick slab waveguide gratings, with emphasis on their resonant properties such as number of resonances, full width at half-maximum, and free spectral range. The slab waveguide gratings with various thicknesses and grating depths are designated and analyzed using a finite-difference time-domain method. Since the slab waveguide is much thicker than the wavelength of operation, a large number of resonances are obtained in the spectral region of interest. By adding an antireflection coating having thickness of quarter-wavelength on the back side of the waveguiding slab, we observe that a usual interference between the directly transmitted zero-order mode and the diffracted order generating the guided mode is eliminated. As a consequence, the sharp resonances and undisturbed comb-like spectra are achieved. The incident and polarization angles, and the surrounding medium’s refractive index influence on the resonant properties are also examined. The guided-mode resonance-based comb-like filters may find applications in optical clocks, optical spectroscopy, optical communications, and lasers. Our systematic investigation provides useful guidelines for designing grating-based frequency comb devices.