The novel composite lithium solid polymer electrolytes (SPEs) composed of polyethylene oxide (PEO) matrix and yttrium oxide (Y2O3) nanofillers were prepared by a solution casting method. The crystal morphology of the SPEs was characterized by polarized optical microscope (POM) and wide-angle X-ray diffraction (WAXD). The induced nucleation and steric hindrance effects of Y2O3 nanofillers result in the increased amount as well as decreased size of PEO spherulites which are closely related to the crystallinity of the SPEs. As the Y2O3 contents increase from 0 wt% to 15 wt%, the crystallinity of the SPEs decreases proportionally. The thermal, mechanical and electrical properties of the SPEs were investigated by thermal gravimetric analysis (TGA), dynamic mechanical analysis (DMA) and AC impedance method, respectively. The physical properties including thermal, mechanical and electrical performances, depending remarkably on the polymer-filler interactions between PEO and Y2O3 nanoparticles, are improved by different degrees with the increase of Y2O3 contents. The (PEO)21LiI/10 wt%Y2O3 composite SPE exhibits the optimal room-temperature ionic conductivity of 5.95×10-5 Scm-1, which satisfies the requirements of the conventional electrochromic devices. The novel composite lithium solid polymer electrolytes (SPEs) composed of polyethylene oxide (PEO) matrix and yttrium oxide (Y2O3) nanofillers were prepared by a solution casting method. The crystal morphology of the SPEs was characterized by polarized optical microscope (POM) and wide -angle X-ray diffraction (WAXD). The induced nucleation and steric hindrance effects of Y2O3 nanofillers result in the increased amount as well as decreased as PEO spherulites which are closely related to the crystallinity of the SPEs. As the Y2O3 contents increase from The thermal, mechanical and electrical properties of the SPEs were investigated by thermal gravimetric analysis (TGA), dynamic mechanical analysis (DMA) and AC impedance method, respectively. The physical properties including thermal, mechanical and electrical performances, may remark remarkably on the polymer-filler interactions between PEO and Y2O3 nanoparticles , which are improved by different degrees with the increase of Y2O3 contents. The (PEO) 21LiI / 10 wt% Y2O3 composite SPE exhibits the optimal room-temperature ionic conductivity of 5.95 × 10-5 Scm-1, which satisfies the requirements of the conventional electrochromic devices.