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A terahertz time-domain spectroscopy (THz-TDS) system was used to monitor the crystallinity and moisture content of polyethylene terephthalate (PET) specimens. The crystallinity and moisture content were varied by controlling cooling rate and immersion time. Crystallinity, crystallite size, and moisture content of the PET specimens were measured through differential scanning calorimetry (DSC), x-ray diffraction (XRD), and gravimetric methods, respectively. The THz signal with regard to crystallinity and moisture content was subsequently analyzed within the time and frequency-domain. In addition, optical properties of refractive index and absorption coefficient were derived. Consequently, the crystallinity and moisture content could be simultaneously measured by analyzing the THz signals.