All solid-state thin film battery was prepared with conventional sputtering technologies. Low conductivity of lithium phosphorus oxynitride (LiPON) electrolyte and higher resistance at the interface of LiCoO2/LiPON was crucial for the development of thin film battery. Presence of thermally treated Al2O3 thin film at the interface of LiCoO2/LiPON decreased the interfacial resistance and increased the discharge capacity with the better cycling behaviors. Surface analysis and electrochemical impedance measurement indicate the formation of solid solution LiCo1−y Al y O2 at the interface of LiCoO2/LiPON.