In this thesis, samples of partially oriented yarn (POY) PET fibers were uniaxially drawn below the glass transition temperature (Tg) at a various draw ratios to obtain filaments with different structures and then were exposed to scCO2 under and without tension-conditions to induce morphological changes in them. A number of studies are reported in the literature on the influence of scCO2 on the structural changes of PET materials. However, no in-depth study has been reported on the influence of scCO2 on the mesomorphic transitions in PET fibers. While exposure of fibers to scCO2 allows for tailoring desirable structural changes in them without any environmental hazards, designing new polymer processes or improving upon old ones can only be achieved by exactly knowing the mechanism of microstructure development and the mesomorphic transitions in the oriented PET fibers exposed to scCO2. The birefringence and density results illustrate that cold-drawing and increasing draw ratio gives rise to an increase in the overall molecular orientation followed by an increase in the molecular compactness in the non-crystalline regions. Analyzing the results of DSC and FTIR showed that the amount of extended chains and then mesomorphic structures have increased during cold-drawing in the amorphous regions. The mechanical properties results show that cold-drawing and increasing draw ratio leads to an increase in the tensile strength and a decrease in the breaking elongation. The reduction of breaking elongation as well as increment of density and crystallinity without any noticeable peaks in WAXS curves illustrated that the molecular extensibility in the non-crystalline regions decreased by increasing the draw ratio. The curves of loss tangent (loss factor) versus temperature from DMA results were resolved into three peaks at above 0 0C temperature range, which indicated the existence of different molecular relaxations and structures in the non-crystalline regions. These structures appear to comprise from complete disorder to oriented amorphous (mesophase) structure. The loss tangent temperature curves below 0 0C did not show any remarkable variations with cold-drawing and also increasing the draw ratio. Our results showed that cold-drawing of PET fibers leads to the formation of a mesomorphic phase in the samples and also different degrees of molecular orientation in the amorphous region with more mesophase and crystals formed in the samples with the increasing draw ratio. Exposure of PET samples to scCO2 leads to absorption of CO2 molecules into the free volumes of amorphous phase. As a result, it gives rise to an increase in the degree of crystallinity followed by a decrease in the mesomorphic structures. It can be claimed that sample treatment conditions have an important role on the structural changes and in transforming the oriented chains in the mesophase into the crystalline or non-crystalline domains. PET fibers exposed to scCO2 under tension present higher values of crystallinity, orientation, tenacity, Young’s modulus and molecular compactness in the amorphous domains but lower values of the breaking elongation, extensibility and transient structures than the samples exposed without tension. Exposure of the samples to scCO2 leads to the transformation of the transient structure mainly into the crystalline phase followed by increasing sample stiffness and storage modulus. When fibers are exposed to scCO2 without tension; some extended chains in the mesophase may be loosened and relaxed. Exposure of the samples to scCO2 without tension leads to an increase in the mobility of the chains and segments in amorphous domains, a situation which is characterized by decreasing values of storage modulus in the samples. As a result, it is possible to study the structural changes in the amorphous phase of these samples below 0 0C. WAXS results showed that for the conditions used in our experiments, cold-drawing and increasing draw ratio, then exposing to scCO2 leads to a reduction in the crystallite size along the chain direction. In contrast, a growth in crystallite sizes was observed in lateral directions. Exposure of the samples to scCO2 under tension, not only higher values of crystallinity are obtained in PET fibers, but also greater crystallite sizes forms in this case than when the samples were exposed without tension. On the basis of these observations, it can be claimed that exposure to scCO2 and also sample treatment conditions have important role on the structural changes and mesomorphic transitions.