In view of creating a foamed PET fiber through a solid foaming process, we investigated the morphological changes of PET yams induced by several methods under various supercritical carbon dioxide (SCCO2) conditions. A special apparatus equipped with a mechanism for drawing fibers under high-pressure atmospheres was constructed. The PET yarn was foamed through the process of adsorption/decompression of SCCO2/1-BuOH mixtures with drawing of the yam. The yarn was found to have different morphologies depending on the draw ratio in the fluid; in cases of low draw ratios, spherical bubbles were generated, and in cases of high draw ratios, crack-like bubbles were generated along the fiber axis. Also, the morphology of the treated yarn was foamed around the center of the yam but not near the yam surface. The specific mechanism of the morphological changes for the PET yarn produced through SCCO2 treatment was suggested from the relation between morphology of the treated yam and the treatment condition. (c) 2006 Elsevier B.V. All rights reserved.