The　vibration　method　is　an　effective　method　used　for　on-line　monitoring　the　power　transformer＇　delitescent　fault,　it　can　monitor　not　only　the　winding,　but　also　the　condition　of　core.　In　this　paper,　a　method　extracting　core＇s　vibration　signal　with　operating　transformer　at　on-load　condition　was　introduced.　The　transformer＇s　vibration　accelerated　signals　at　different　on-load　current　and　no-load　condition　were　measured.　From　the　fitted　curve　of　on-load　current　versus　fundamental　frequency　component　of　vibration　accelerated　signal,　the　value　of　fundamental　frequency　component　was　gained　as　the　on-load　current　was　zero,　and　it　was　almost　equivalent　to　the　fundamental　frequency　component　of　vibration　accelerated　signal　measured　at　no-load　condition,　thus　using　on-load　current　method　the　fundamental　frequency　component　of　core＇s　vibration　can　be　acquired.　The　core　with　fault　was　finally　monitored　using　the　on-load　current　method　compared　with　WPT　method.　From　the　result　of　experiment,　we　can　see　this　method　is　very　useful　to　estimate　the　condition　of　the　transformer＇　core.