All-solid-state lithium ion battery (LIB) has raised attention as the next-generation LIB for possibilities of high safety and energy density. We invented composite electrolytes consists of polymer nanofibers and PEO of typical polymer electrolytes and realized improved ionic conductivity, Li+ transference number and thinner membrane. We have studied composite electrolytes with Li+ conductive nanofiber introduced PEO side chain and highly polarized PVDF nanofiber with high ratio of βcrystal respectively and the composite electrolyte with hi salt concentration and reported improvements of electrolytes property due to the interaction between nanofiber and polymer matrix. Currently, we’re also working on development of cathodes which is suitable for solid polymer electrolytes.