The primary focus of this work was to evaluate the relationship between free volume distribution, chain motion, crystallinity, and the gas properties of PET upon strain-induced crystallization (SIC) at different stretch ratios. The formation of a three-phase structure containing rigid amorphous phase, mobile amorphous phase, and crystalline phase upon SIC was confirmed by differential scanning calorimetry and positron annihilation lifetime spectroscopy (PALS). Dynamic mechanical analysis and PALS indicated that there was a significant reduction in the fractional free volume upon orientation at an extension ratio of 3 x 3. Sub-T-g relaxation studies indicated that the activation energy of mechanical relaxation decreased with increasing the stretch ratio. Gas transport studies revealed that the reduction in permeability coefficient was mainly due to reduction in diffusivity. Permeation studies using gas molecules with different sizes revealed that strain-induced crystallization affects the free volume distribution as well as reducing the average fractional free volume.