For application to all-solid-state thin film batteries, lithium boron phosphorus oxynitride (LiBPON) thin films were fabricated by RF magnetron sputtering technique of lithium borate (Li3BO3) and lithium phosphate (Li3BO4) target under N-2 atmosphere. To investigate the mixed former effect upon the LiBPON thin films, we prepared various films with controlled B and P contents. The LiBPON thin film formulation with nearly equal B and P contents showed the highest room-temperature lithium ionic conductivity of 3.52 x 10(-6)S cm(-1) and the lowest activation energy of 48.24 kJ mol(-1) owing to the mixed former effect, the increased lithium content and nitridation. Infrared spectra of the glass thin films achieved two dominant peaks corresponding to BO33- and PO43- unit. N1s spectra were analyzed by X-ray photoelectron spectroscopy that the increased ionic conductivities observed were related to the formation of triply coordinated nitrogen.