The structural relaxation of an atactic polystyrene under sub-Tgannealing at 50°C and at 70°C was investigated by the radial distribution function (RDF) derived from its wide-angle x-ray scattering (WAXS). By recording the changes in RDF (i.e., ΔH(r), whereris the radial distance from an arbitrary reference atom) after it had been annealed for a certain period of days, and taking the integration of ΔAH(r)2dr, we found that the atom density within structural domains in a size below 15 Å was changed dramatically, but that the tendency leveled off as annealing proceeded. However, the atom density outside the domains is barely changed by sub-Tgannealing. The size of the domain is similar to the statistical segment length reported in the literature. The behavior of the domains—that the segmental relaxation inside the domains in the initial sub-Tgannealing is unconstrained by their neighbors outside the domains—is also similar to the behavior of the statistical segments. The domains are believed to be composed of a statistical segment across the center, which has about 6 styrene repeat units, and 6 equal-distanced parallel segments tangent to the edge of the domain. On the other hand, as the annealing temperature is closer to the glass transition temperature, the frozen unstable chain segments reach equilibrium sooner and with less disturbance in their conformation.