Small-angle X-ray scattering (SAXS) is employed to study the supermolecular structure of uniaxially oriented and semicrystalline poly(ethylene terephthalate) (PET) samples. By means of a multistep zone drawing procedure carried out at different temperatures and loading conditions an extraordinarily high chain orientation is achieved. Using the electron density correlation function obtained from the Fourier transformed experimental scattering intensity, structure parameters such as the most probable long period, the average lamellar thickness, and the volume crystallinity are determined. By obtaining these parameters from only one experiment one can neglect possible effects from a combination of methods and thus gain new insights into the structure formation process under drawing.