Return　voltage　measurement　(RVM)　is　a　non-destructive　method　to　diagnose　insulation　condition　of　power　transformers.　In　order　to　make　full　use　of　the　advantages　of　the　recovery　voltage　method　in　the　oil-paper　insulation　diagnosis,　we　put　forward　a　method　of　using　the　gray　relational　model　to　diagnose　insulation　condition　of　power　transformers.　First,　RVM　is　used　to　extract　the　characteristics　which　can　reflect　insulation　condition　of　power　transformers.　Then,　a　large　amount　of　test　data　are　used　to　establish　a　standard　state　vector.　Finally,　the　gray　relational　diagnostic　model　is　used　to　diagnose　insulation　condition　of　power　transformers.　According　to　the　grey　correlation　degree　between　diagnosing　transformers　and　standard　state,　the　insulation　condition　of　power　transformers　can　be　diagnosed.　Evaluation　results　show　that　the　diagnosis　results　are　in　accordance　with　the　actual　insulation　state　of　transformer.　©　2015,　High　Voltage　Engineering　Editorial　Department　of　CEPRI.　All　right　reserved.