The effects of nanosized ZnO on the microstructure, the free volume and the ionic conductivity of poly ethylene oxide (PEO) nanocomposite electrolytes (PEO)8-ZnO-LiClO4 are systematically studied by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and positron annihilation lifetime spectroscopy (PALS), respectively. The experimental results show that the presence of the nanosized ZnO brings about a reduction in the crystallinity of the PEO phase and a very marked increase in ionic conductivity. PALS discrete analysis shows that the free volume size, the free volume concentration and the relative free volume fraction significantly increase with the increase of nano-ZnO. Specially, it is the first time to observe the occurrence of peaks of the free volume distribution split after nano-ZnO has been filled, which indicates that the ZnO added into (PEO)8-LiClO4 has an important effect on the microstructure for nanocomposites due to the interaction between the nano-ZnO and matrix. A direct correlation between the free volume fraction and the ionic conductivity is observed, and the ionic conductivity mechanism is discussed.