An all-solid-state thin-film battery consisting of a c-axis-oriented LiCoO2 thin-film and a lithium phosphorus oxynitride (LiPON) glass electrolyte was fabricated. Thermal treatment at 473K after fabrication of the LiPON/LiCoO2 interface decreased the charge transfer resistance at the interface, and the resistance was further reduced by prolonging the thermal treatment time. The charge transfer resistance per unit electrode area (interfacial resistivity) of a film battery thermal-treated for 60min decreased down to 125Ωcm2, which is ca. five times larger than that in the case of an organic electrolyte (1moldm−3 LiClO4 dissolved in propylene carbonate)/LiCoO2 interface (25Ωcm2). Due to the reduction of the charge transfer resistance at the LiPON/LiCoO2 interface, the reaction current of the film battery was greatly increased by the thermal treatment. Also, thermally treated film batteries showed stable electrochemical lithium insertion/extraction properties compared with the batteries using conventional organic electrolytes. Both the voltammograms and the impedance spectra of the film battery maintained their initial shape for over 100 cycles, and the capacity retention ratio per cycle was calculated to be 99.9%.