Abstract: In order to investigate the effects of polyethylene glycol (PEG) on the properties of cellulose nanocrystals (CNCs)/poly 3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) composites, polyethylene glycol (PEG)-CNCs/PHBV composites were prepared by the melt compounding method. The facture structure, crystalline properties, thermal stability, and mechanical properties of PEG-CNCs/PHBV composites were charactered by ESEM, DSC, POM, TG and material-testing machine, respectively. The results show that the cross section of CNCs/PHBV composites with the addition of PEG becomes rough and fracture suface becomes uneven. The addition of PEG make the spherulite sizes of PEG-CNCs/PHBV composites decrease, new spherulite structure defects emerge, and melting process transform into two melting peaks. The melt temperature T_m decreases from 167.8℃ to 165.1℃. In addition, the addition of PEG enhances the mobility of PHBV molecular chain, thus the crystallization of PHBV becomes more difficult. The melt crystalline temperature T_mc of composites decreases from 99.8℃ to 73.5℃ and the crystallinity X_c decreases from 54.3% to 50.2%. Compared with PHBV, the impact strength and tensile elongation at break of PEG-CNCs/PHBV composites keep increasing with the increase of PEG content, whose maximal promotion with 25wt% PEG are 56.4% and 96.3%, respectively, but the Young's modulus and tensile strength of PEG-CNCs/PHBV composites keep decreasing. Meanwhile the application of PEG improves the thermal stability of composite materials. The pyrolysis process of PEG-CNCs/PHBV composites transforms into two steps, and the fastest decomposition temperature (T_max1) in the first and second step increases from 281.5℃ to 285.3℃, and from 371.5℃ to 394.3℃, respectively. The addition of PEG can improve the compatibilization and crystallinity between CNCs and PHBV, accordingly enhance toughness, plasticity and thermal stabilities of CNCs/PHBV composites.