To　improve　the　morphology　of　carbides　in　carbidic　ductile　iron,　a　compound　modifier　consisting　of　0.1%　Nb　0.1%　Ti　+/-　0.1　wt.%　Y　was　added　to　the　base　ductile　iron　with　chemical　composition　of　3.72%　C,　2.77%　Si,　0.51%　Mn,　0.99%　Cr　and　balance　Fe　(wt.%).　The　effect　of　this　compound　modifier　on　the　microstructures　of　carbidic　ductile　iron　was　studied.　Also,　first-principles　calculations　were　carried　out　to　better　understand　the　modification　mechanisms.　The　results　showed　that　the　maximum　diameter　of　spheroidal　graphite　nodules　decreased　from　58　to　34　mu　m　after　the　addition　of　compound　modifier,　and　continuous　carbide　networks　changed　into　a　broken　network.　The　roundness　of　graphite　nodules　decreased　slightly,　and　the　percent　nodularity　of　the　graphite　nodules　and　the　number　of　carbides　decreased　by　3　and　1.8%,　respectively.　Compounds　with　higher　melting　point　are　formed　thanks　to　the　compound　modifier　which　acts　as　heterogeneous　core,　and　the　remaining　Ti　and　Nb　elements　can　be　selectively　attracted　by　(010)　surface　of　Fe8Cr4C4.　Furthermore,　Cr　elements　can　be　easily　replaced　by　Ti　and　Nb　in　the　carbides　to　form　more　stable　Fe8Cr3TiC4　and　Fe8Cr3NbC4,　which　can　prevent　the　continuing　growth　of　carbide　on　the　Fe8Cr4C4　(010)　crystal　surface　and　break　the　continuous　network　M3C.　Y　atoms　cannot　be　directly　adsorbed　onto　Fe8Cr4C4　(010)　surfaces.　They　combine　first　with　oxygen　in　the　ductile　iron　to　form　Y2O3.　The　work　of　adhesion　of　the　interface　between　a　Y2O3　(100)　and　a　Fe8Cr4C3　(010)　is　predicted　to　be　0.3　J/m(2).　The　addition　of　Y　element　is　found　to　have　a　positive　effect　on　breaking　up　the　continuity　of　the　carbide　network.