The purpose of this study is to investigate the effect of carbon dioxide (CO2) on the crystallization behavior and the mechanical properties of PLLA/PMMA blends with various weight fraction of PMMA. PLLA/PMMA blends can be crystallized even at a low temperature of 0 degrees C under high-pressure CO2. The films treated with high-pressure CO2 at 0 degrees C have about three times larger strain at break than that of the amorphous and cold-crystallized film. The size of spherulites in the CO2 treated film is considered to be smaller than the wavelength of the visible light because of a good transparency. The improvement of the strain at break is attributed to the reduction of the stress concentration during the deformation. (c) 2006 Elsevier Ltd. All rights reserved.