A polycrystalline thin film diffractometer for asymmetric diffraction at a fixed incident angle using parallel beam and high-resolution-parallel slits has been developed. Well collimated CuK α1 incident beam was obtained by coupling of a rotating anode generator with a channel-cut crystal monochromator. The diffracted beam was analyzed with parallel slits with 0.17° angular aperture. The observed profiles of CeO2 standard powder sample were analyzed by using an individual profile fitting technique. The minimum full-width at half-maximum (FWHM) was 0.15° in the 20 range of 40° ∼ 70°, and the variation of FWHM with 20 was almost independent of the incident angle. Intensity in asymmetric diffraction was enhanced compared to that in the symmetric 0-20 scan, and its variation was well in accordance with the theory. The use of the parallel slits compromizes the intensity and resolution for polycrystalline thin film diffraction in the present setup, but gives the high quality of diffraction data for structural study.