The relaxation behavior of neat PET and PET/SiO2 nanocomposites was investigated by using methods of internal friction and DSC.The activation energy of β relaxation was calculated by Arrhenius equation because it was a Debye peak，but the α relaxation didn’t follow the Arrhenius equation，so its activation parameter was calculated by VFT equation.The heights of the α and β peaks were obtained by the results of internal friction measurements.The results showed that the heights of peak，activation parameters of the α relaxation and activation energy of the β relaxation increased with increasing the content of filler.On one hand，SiO2 nanoparticles had grafting reaction with carboxyl，so the motion of polymer chains and groups was more and more difficult with increasing the content of filler.On the other hand，the nanoparticles acted as nucleating point，so they could promote the nucleation and crystallization of PET

and the amorphous regions decreased.The peak heights of the α and β relaxations were related to the normal amorphous regions，so both of them obviously decreased with increasing the content of filler.Hence the relaxation properties of PET chains and groups were changed by addition of SiO2 nanoparticles.