The mixed former effect of boron and phosphorus is applied to improve chemical stability and electrochemical property of thin film electrolyte based on lithium phosphorus oxynitride (LiPON). This thin film with new composition ‘lithium borophosphate oxynitride (LiBPON)’ is deposited by RF magnetron sputtering with double targets of Li3PO4 and Li3BO4. When boron content is varied by controlling the RF power on Li3PO4 and Li3BO4 targets, boron ions successfully replace the four-coordinated phosphorus. Raman spectra and N 1s XPS spectra of this new electrolyte reveal that three-coordinated nitrogen atoms (PN<P) are dominant in glass network, which distinguishes from LiPON films without the BP mixed former effect. The improved electrochemical properties and chemical stability of LiBPON film endow all-solid-state thin film cells (LiCoO2/LiBPON/Li) excellent cycling performances with the initial capacity of 34.5μAhcm−2 and the cycling efficiency and 95.3% of its initial capacity.