Small-angle X-ray scattering (SAXS) has been used to investigate the supramolecular structures of complexes formed by polyethylenimine (PEI) interacting with oppositely charged surfactants of sodium alkyl sulfate (SCnS). The effects of the alkyl chain length of the surfactants, the pH values of complex formation between PEI and SCnS solutions, the molecular weight of branch PEI chains, and the molecular architecture of the polyelectrolyte chains have been examined. By varying these parameters for complex formation, very rich highly ordered nanostructures could be produced. The decrease in the alkyl chain length of surfactants induced a structural transition from densely packed Lamellar I to 2D distorted hexagonal packing of ribbons, then to loosely packed Lamellar II in which the alkyl chains of the surfactant were fully extended and perpendicular to the lamellar surface, and finally to a 2D hexagonal packing of cylinders. The increase in pH values of the complex formation induced a similar structural transition from a 2D rectangular face-centered (or 2D distorted hexagonal) packing of deformed cylinders to Lamellar II, and then to 2D hexagonal packing of cylinders. Both the decrease in the charge density of the PEI chains (caused by an increase in pH) and the surfactant tail length increased the ratio of the area of polar groups to the volume of alkyl chains of CnS ions in the complexes. The structures of complexes formed by branch PEI having different molecular weights and SCnS were compared. The packing of surfactants bound by high molecular weight branch PEI chains was less ordered in comparison with that bound by low molecular weight branch PEI chains, possibly due to the space hindrance and the rigidity of the large branch PEI chains. Three different nanostructures of Lamellar II, 2D hexagonal packing of cylinders, and Ia (3) over bard cubic were, respectively, observed when branch PEI, linear PEI and linear polyvinylamine (PVA) with similar molecular weights were used to complex with surfactants of SC9S and SC10S. Different chain architectures generated different charge densities and space hindrances, thereby constraining the bound surfactants in different arrangements.