No systematic study has been reported on the lamellar thickening in atactic poly(acrylonitrile) (PAN) upon annealing because PAN, in the form of solution-cast films or their drawn products, generally shows no small-angle X-ray scattering (SAXS) maximum corresponding to the lamellar thickness. In this work, PAN crystals were precipitated during the thermal polymerization of acrylonitrile in solution. The nascent PAN film, obtained by the filtration of the crystal suspension, exhibited a clear SAXS maximum revealing the lamellar structure. The lamellar thickening upon annealing of the nascent PAN films was studied in the temperature range 100-180 degrees C, where the degradation was minimal, as confirmed by the absence of an IR absorption band at 1605 cm(-1) ascribed to the cyclized nitrile groups. Above 190 degrees C, the degradation of the samples was significant, and the SAXS became too broad to determine the scattering maximum. The long period was significantly affected by the annealing time (t(alpha)) and the temperature (T-alpha). Depending on t(alpha), three stages were observed for the lamellar thickening behavior. The lamellar thickness stayed constant in stage I (t(alpha) = 0.53 min, depending on T-alpha), rapidly increased in stage II (t(alpha) = 0.5-8 min), and stayed at a constant value characteristic for each T-alpha at yet longer t(alpha)'s in stage III. The lamellar thickness characteristic for T-alpha increased rapidly with increasing T-a at 165 degrees C (or higher), which was 152 degrees C lower than the estimated melting temperature of PAN (T-m = 317 degrees C). A possible mechanism for such lamellar thickening in PAN far below the T-m is discussed on the basis of the enhanced chain mobility in the crystalline phase above the crystal/crystal reversible transition at 165-170 degrees C detected by differential scanning calorimetry and wide-angle X-ray diffraction. The structural changes associated with annealing are also discussed.