The　Bi0.5Na0.5TiO3-based　ferroelectric　is　one　of　the　most　promising　candidates　for　environment-friendly　lead-free　ferroelectric/piezoelectric　materials　for　various　applications　such　as　actuators　and　micro-electromechanical　systems.　The　understanding　and　tailoring　of　the　ferro-(piezo-)electric　properties　of　thin　films,　however,　are　strongly　hindered　by　the　formation　of　the　defects　such　as　dislocations,　ion　vacancies　in　the　film,　as　well　as　by　the　complexity　of　the　interface　between　the　film　and　the　substrate.　An　ideal　system　for　the　study　of　the　polarization　behavior　in　the　ferro-(piezo-)electric　film　would　be　a　fully　matched　system.　In　this　work,　monocrystalline　0.89Bi(0.5)Na(0.5)TiO(3)-0.11BaTiO(3)　thin　films　were　epitaxially　grown　on　(001)-oriented　Nb-doped　SrTiO3　substrates　using　a　sol　gel　technique.　The　films　were　almost　fully　lattice-　and　thermally　matched　with　the　substrate,　thus　avoiding　the　impact　of　dislocations　and　thermal　stress.　The　films　were　self-poled　by　a　built-in　electric　field,　originating　from　the　sedimentation　of　heavier　atoms　during　the　film　preparation.　As　a　consequence,　an　upward　self-polarization　was　introduced　into　the　films,　giving　rise　to　asymmetric　phase　hysteresis　loops　and　domain　switching　current　responses.　These　results　highlight　the　importance　of　the　interface　complexity　for　the　self-polarization　of　piezoelectric　thin　films,　even　for　fully　matched　films,　which　will　therefore　facilitate　the　control　of　properties　of　piezoelectric　films　and　their　applications　for　various　functional　devices.