Return　voltage　method　(RVM)　is　a　good　method　to　study　aging　state　of　transformer　insulation,　but　it　is　difficult　to　accurately　assess　the　insulation　aging　state　by　a　single　characteristic　quantity.　In　this　paper,　the　fuzzy　rough　sets　theory　combined　with　RVM　is　proposed　to　assess　the　oil　paper　insulation　state　of　transformer　and　construct　the　assessment　system　of　oil　paper　insulation　of　transformer　based　on　a　lot　of　test　data.　First,　the　evaluation　index　of　oil-paper　insulation　status　of　transformer　is　established　by　return　voltage　characteristic　parameters.　Then,　fuzzy　c-means　clustering　algorithm　is　used　to　obtain　the　membership　function　of　the　transformer　test　data　along　with　fuzzy　partition　of　characteristics　.Moreover,　the　fuzzy　attributes　of　assessment　table　of　oil　paper　insulation　statue　is　simplified　according　to　the　distinct　matrix,and　it　extracts　the　evaluation　rule　of　oil　paper　insulation　condition.　Finally,　the　examples　in　this　paper　demonstrate　that　the　assessment　system　is　effective　and　feasible,　which　provides　a　new　idea　for　the　assessment　of　transformer　oil-paper　insulation　state.　The　research　has　practical　value　in　application　of　engineering.　©　2016　The　authors　and　IOS　Press.　All　rights　reserved.