This article presents foaming behaviors and cellular structures of poly(lactic acid) (PLA)-clay nanocomposite and respective neat PLA foams using CO2 as a foaming agent. The effects of nanoclay and foaming conditions on the foam morphology and mechanical properties of neat (PLA) and PLA-clay nanocomposites have been investigated by temperature modulated differential scanning calorimeter (TMDSC), X-ray diffraction, scanning electron microscopy, and tensile properties. CO2-induced crystallization in PLA and PLA-clay nanocomposites and the crystallinity increased with an increase in saturation time. The present of clay acted as nucleation site to facilitate the crystallization process of PLA thus resulted in smaller crystallites and lower crystallinity. Unique skin-core foam morphologies in PLA and PLA-clay nanocomposites were obtained by controlling the crystallinity. Clay nanoparticles served as nucleation sites to generate foam morphology in PLA-clay sample with high crystallinity. The introduction of the cellular structure improved the mechanical strength of the material and the specific strength of PLA foams were increased in the presence of a small amount of MMT. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012