The　effect　of　coherency　WC/Co　phase　boundaries　on　the　fracture　toughness　of　the　nanocrystalline　WC-Co　cemented　carbides　is　studied　by　MD　simulation　method.　The　simulation　results　show　that　the　nanocrystalline　WC-Co　cemented　carbides　with　coherency　WC/Co　phase　boundaries　has　higher　fracture　toughness　than　that　without　coherency　WC/Co　phase　boundaries.　Moreover,　the　mechanism　of　why　coherency　WC/Co　phase　boundaries　can　improve　the　fracture　toughness　of　the　nanocrystalline　cemented　carbides　is　also　investigated.　It　is　found　the　fact　that　the　separation　energy　of　the　coherent　WC/Co　phase　boundary　is　larger　than　that　of　the　incoherent　WC/Co　phase　boundaries　is　the　main　reason　for　this　excellent　mechanical　property.