The　capacitance　dispersion　in　La　and　Mg　co-substituted　BiFeO3　thin　film　has　been　studied　at　different　stages　of　polarization　switching.　A　negative　capacitance　(NC)　behavior　is　observed　in　the　sample　that　is　fatigued　above　10(9)　switching　cycles.　The　origin　of　the　NC　is　investigated　through　analyzing　relaxation　processes　and　charge　transport　kinetics　by　admittance　spectroscopy.　An　activation　energy　of　similar　to　0.6　eV　and　a　zero　field　mobility　mu(0)　=　5.33　+/-　0.02　x　10(-13)　m(2)/Vs　are　thus　obtained.　A　physical　mechanism　is　proposed　to　explain　this　behavior.　It　involves　a　redistribution　of　oxygen　vacancies,　which　are　trapped　at　the　film/electrode　interface　during　the　fatigue　process.　(C)　2012　American　Institute　of　Physics.　[http://dx.doi.org/10.1063/1.4733982]