In order to improve the crystallization properties of the polyethylene terephthalate (PET), nano-zinc oxide (nano-ZnO)/PET and nano-ZnO/polyethylene glycol (PEG)/PET composites were successfully synthesized by in-situ polymerization method in this work. The experimental results indicated that nano particles can accelerate the crystallization of the PET which was demonstrated by increase in the intensities of the XRD peaks. PEG and PET form copolymer in composites, leading to the improvement of the flexibility of the molecule chain. Meanwhile, PEG was able to encapsulate the nano particles to make them have both good compatibility to matrix and uniform dispersion in the composites. Therefore, the crystallization rate and crystallization capability of nano-ZnO/PEG/PET composites were reasonably raised.