The aim of this study was to investigate the crystallization behavior and UV-protection property of polyethylene terephthalate (PET)-ZnO nanocomposits. PET-ZnO nanocomposites containing 0.5-3.0 wt % of ZnO were successfully synthesized by in situ polymerization. The Fourier transformed infrared (FTIR) spectroscopy indicated the silane coupling agent was anchored onto the surface of ZnO. Scanning electron microscope (SEM) images showed ZnO particles were dispersed homogeneously in PET matrix with amount of 0.5-1.0 wt%. Differential scanning calorimetry (DSC) results exhibited that the incorporation of ZnO into PET resulted in increase of the melting transition temperature (T-m) and crystallization temperature (T-c) of PET-ZnO nanocomposites. The crystallization behavior of PET and PET-ZnO nanocomposites was strongly affected by cooling rate. ZnO nanoparticles can act as an efficient nucleating agent to facilitate PET crystallization. UV-vis spectrophotometry showed that UV-ray transmittance of PET-ZnO nanocomposites decreased remarkably and reached the minimum value of 14.3% with 1.5 wt % of ZnO, compared with pure PET whose UV-ray transmittance was 84.5%. PET-ZnO nanocomposites exhibited better UV-protection property than pure PET, especially in the range of UVA. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 1303-1311, 2009