Exfoliated poly(ethylene terephthalate) (PET)/organoclay nanocomposites were prepared by extrusion blending method with different clay contents and molecular weights, of raw PET. Pyromellitic dianhydride (PMDA) was extruded, together with PET and day, to introduce long-chain branching to PET backbone and delaminate the,clay layers. Although the molecular weights and viscoelastic properties of nanocomposites were much lower than those of foamable PMDA-modified PET, the melt foamability of nanocomposites was significantly improved by the well-dispersed clays due to the heterogeneous nucleation effect, enhanced nonisothermal crystallization rate, and so on. Foaming temperature windows with a width of 20-60 degrees C Were explored for PET/day nanocomposites with intrinsic viscosities of 0.67-0.94 dL/g,, in which nanocomposites foams with cell diameters of 29-53 mu m, cell densities of 6.5 x 10(7)-6.9 x 10(8) cells/cm(3), and expansion ratio of 10-50 could be controllably produced using supercritical CO2 as a blowing agent.