The origin of the orientation dependence in electron diffraction from a discrete helix or a helical structure in general has been studied with the development of intuitive algebraic expressions of the intensity distributions in the diffraction space. It is shown that there is strong orientation dependence of diffraction intensities, which can even result in certain layer lines missing when at least two dominating Bessel functions interfere at these layer lines. The cases have been analyzed that electron diffraction patterns from chiral, non-chiral single-walled carbon nanotubes (SWCNTs) and B-DNA, respectively, show obvious orientation dependence and extinction of layer lines. (c) 2005 Elsevier B.V. All rights reserved.