Elastic small-angle scattering of X-rays, neutrons, and light from an assembly of hexagonally packed cylindrical particles with a paracrystalline distortion was investigated. The scattering formula is presented, and the effects of the paracrystalline distortion factor g and volume fraction of the particles on the scattering profiles are discussed. This scattering theory is applied to small-angle X-ray scattering (SAXS) from a binary mixture of a sphere-forming block copolymer and a lamellar-forming block copolymer. The SAXS study confirmed that the two copolymers are mixed at the molecular level, forming the hexagonally packed cylindrical morphology, and the various characteristic parameters describing the morphology were extracted from the best fit between the experimental and theoretical scattering profiles. The SAXS results were confirmed by transmission electron microscopy.