Fatigue　failure　in　ferroelectrics　has　been　intensively　investigated　in　the　past　few　decades.　Most　of　the　mechanisms　discussed　for　ferroelectric　fatigue　have　been　built　on　the　＂hypothesis　of　variation　in　charged　defects＂,　but　these　are　rarely　evidenced　by　experimental　observation.　Here,　using　a　combination　of　complex　impedance　spectra　techniques,　piezoresponse　force　microscopy　and　first-principles　theory,　we　examine　the　microscopic　evolution　and　redistribution　of　charged　defects　during　the　electrical　cycling　in　BiFeO3　thin　films.　The　dynamic　formation　and　melting　behaviors　of　oxygen　vacancy　(V-O)　order　are　identified　during　the　fatigue　process.　It　reveals　that　the　isolated　V-O　tends　to　self-order　along　grain　boundaries　to　form　a　planar-aligned　structure,　which　blocks　the　domain　reversals.　Upon　further　electrical　cycling,　migration　of　V-O　within　vacancy　clusters　is　accommodated　with　a　lower　energy　barrier　(similar　to　0.2　eV)　and　facilitates　the　formation　of　a　nearby-electrode　layer　incorporated　with　highly　concentrated　V-O.　The　interplay　between　the　macroscopic　fatigue　and　microscopic　evolution　of　charged　defects　clearly　demonstrates　the　role　of　ordered　V-O　clusters　in　the　fatigue　failure　of　BiFeO3　thin　films.　(C)　2014　Acta　Materialia　Inc.　Published　by　Elsevier　Ltd.　All　rights　reserved.