We　have　studied　the　polarization　fatigue　of　La　and　Mg　co-substituted　BiFeO3　thin　film,　where　a　polarization　peak　is　observed　during　the　fatigue　process.　The　origin　of　such　anomalous　behavior　is　analyzed　on　the　basis　of　the　defect　evolution　using　temperature-dependent　impedance　spectroscopy.　It　shows　that　the　motion　of　oxygen　vacancies　(V-O(center　dot　center　dot))　is　associated　with　a　lower　energy　barrier,　accompanied　by　the　injection　of　electrons　into　the　film　during　the　fatigue　process.　A　qualitative　model　is　proposed　to　explain　the　fatigue　behavior,　which　involves　the　modification　of　the　Schottky　barrier　upon　the　accumulation　of　V-O(center　dot　center　dot)　at　the　metal-dielectric　interface.　(C)　2012　American　Institute　of　Physics.　[doi:　10.1063/1.3678636]