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We report on one-dimensional photonic crystal slabs designed to serve as narrowband reflection filters at terahertz frequencies.The reflection spectra of photonic crystal slabs exhibit sharp resonant features with Fano line shapes due to coupling of the leaky photonic crystal modes to the continuum of free-space modes.We adopt these resonances to achieve filtering capability.Actually, one-dimensional photonic crystal slabs can be used as volumetric quasi-optical elements for a broad frequency range.Compared with optical photonic crystal slabs, the terahertz photonic crystal slabs are easy to fabricate due to longer working wavelength.In this paper, we give the method to position the central frequency at choice in light of the dispersion curves of leaky modes and diffracted waves.Bandwidth as narrow as several gigahertz at halfmaximum is illustrated.Symmetric response and sideband suppression are achieved using the thin film effect.At last, we design an angle-tunable, oblique incidence resonant filter.In the region of interest, the shift of thepeak frequency is linear with respect to changes in the angle of incidence, which provides convenient tuning capability.