Samples of polycarbonate (PC) before and after remelting and isothermal solidification treatment were characterized by wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS), microhardness, and stretching calorimetry techniques. Both PC samples could be regarded as structurally identical (X-ray amorphous); however, the isothermally solidified sample exhibited higher microhardness and Young's modulus, as well as a lower linear expansion coefficient and greatly reduced breaking strain. These results are attributed to the overlap of a network of ultramicrocrystals on the original entanglement network in the reference (untreated) PC sample.