The degree of crystallinity in industrial Polyethylene Terephthalate (PET) has major effects on the physical and mechanical properties of the polymer when used in films, fibers, and molded parts, Crystallinity of injection molded and heat treated PET bars was investigated using x-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), differential scanning calorimetry (DSC), and the material density technique. The data showed that amorphous PET starts to crystallize at similar to 105 degreesC and, while both XRD and FTIR are consistent and reliable techniques in quantifying crystallinity, the FTIR method is more efficient in terms of instrument operation and data analysis.