The morphological development during crystallization and its subsequent effect on the relaxation behaviour for a series of cold-crystallized samples of a commercial polyimide, Mitsui Toatsu＇s New TPI, has been studied. Small angle X-ray scattering (SAXS) results showed that both the average lamellar and amorphous layer thicknesses increased with increasing crystallization temperature; the degree of crystallinity was also shown to increase with increasing crystallization temperature by wide angle X-ray scattering (WAXS). The glass transition temperature, however, was shown to decrease with increasing crystallization temperature. Increasing crystallization time at a given crystallization temperature produced an increase in T-g, with the increase following a log time dependence. An interesting feature observed from the dynamic mechanical studies was that for low crystallization times, the dependence of T-g On crystallization time was different from that for higher crystallization times, the changeover time corresponding to spherulitic impingement. The degree of crystallinity was shown to increase with increasing crystallization time; at low crystallization times, a prominent increase was observed due to a primary crystallization process, following which a further gradual increase in crystallinity was observed, corresponding to a secondary crystallization process. This was accompanied by a decrease in the average lamellar thickness; the average amorphous layer thickness, however, remained unchanged. This has been explained on the basis of a secondary crystallization process leading to the formation of thinner lamellae in the amorphous regions between different lamellar stacks, which gives rise to the lower melting endotherm (T-c + 10) observed in d.s.c. scans.