Block-graft copolymer, poly(St)-block-[poly(HSt)-graft-poly(EO)]-block-poly(St) was prepared from styrene (St), p-tert-butoxystyrene (BuOSt), and ethylene oxide (EO) in four steps through anionic polymerization: (1) block copolymerization of St, BuOSt, and St; (2) deprotection of tert-butoxy group; (3) potassiation of phenolic OH; (4) graft copolymerization of EO initiated from phenoxide anion. The samples obtained were confirmed to have predictable number and length of graft chains as well as predictable length of backbone triblock copolymer and narrow molecular weight distributions by precise characterization using osmometry, H-1 and C-13-NMR. Th,,ample showed very clear microphase separation structure and the sample/LiClO4 complex showed high ionic conductivity. The block-graft copolymer is expected to be a new class of material for solid polymer electrolyte.