A novel polymer electrolyte having borate ester groups, which are fixed to the backbone chain of the polymer, was prepared. The backbone polymer was synthesized by reaction between polyethylene glycol and boric acid anhydride, The highest conductivity was found for the polymer electrolyte sample prepared by the polyethylene glycol having average molecular weight of 600 (PEG600), the values of the ionic conductivity were 5.8 x 10(-5) S cm(-1) at 30 degreesC and 2.6 x 10(-4) S cm(-1) at 60 degreesC, respectively. The solid polymer electrolytes have relatively high thermal stability and electrochemical stability.