Theoretical equations for the small-angle scattering of double-strand helixes with fanlike and round cross sections, as well as a cylinder, were applied to determine the geometry of the self-assembled helical structure formed by N-octadecanoyl-L-aspartic acid (C(18)Asp). However, those already-known equations were not sufficient to reproduce the experimental small-angle neutron scattering (SANS) curves. Then, a theoretical equation to the small-angle scattering for a helical fiber was derived. The equation with 5-fold integration covers the available equations for fibers with finite cross sections, but it is generalized so as to suit the multistrand helical structures with arbitrary cross-sectional shapes. Computer simulations based on this theory showed that the best fit to the observed SANS data of C(18)Asp fiber is obtained for a model of a single-strand helix with rectangular cross section, that is, the twisted ribbon.