An　improved　IEC-60599　three-ratio　fault　diagnosis　technique　for　power　transformer　is　proposed,　which　is　based　on　the　Rough　Set　Theory.　Firstly,　a　logical　diagnostic　table　of　IEC-60599　is　presented,　and　a　decision　table　can　be　constructed　by　applying　the　rough　set　theory　based　on　the　IEC-60599　three-ratio　fault　diagnosis　table.　Then,　the　reduction　of　the　decision　table　will　be　obtained,　and　an　improved　new　IEC-60599　three-ratio　fault　diagnosis　decision　table　for　transformer　fault　diagnosis　can　be　acquired.　The　results　of　the　experiment　demonstrate　that　the　improved　new　IEC-60599　three-ratio　fault　diagnosis　technique　extends　the　coding　area　of　the　earlier　IEC-60599　three-ratio　fault　diagnosis　technique,　the　actual　malfunction　diagnosis　ability　is　proved　and　fault　diagnosis　accuracy　is　higher　compared　with　the　earlier　IEC-60599　three-ratio　diagnosis　technique　at　the　incompleteness　and　complexity　of　fault　diagnosis　for　power　transformer.　The　method　is　easily　used　to　engineering　technician.　All　these　properties　can　be　applied　in　practice.