Three-dimensional　numerical　simulation　of　the　deformation　and　breakup　of　an　isolated　liquid　drop　suspended　in　immiscible　viscous　fluid　under　shear　flow　is　performed　with　diffuse　interface　method.　The　governing　equations　of　the　model　are　described　by　Navier-Stokes-Cahn-Hilliard　equations.　The　surface　tension　is　incorporated　into　the　model　through　a　modified　stress.　The　uniform　staggered　Cartesian　grid　is　used.　The　transient　Navier-Stokes　equations　are　solved　by　approximation　projection　method　based　on　pressure　increment　formulation.　The　Cahn-Hilliard　equations　are　solved　by　nonlinear　full　approximation　multi-grid　method.　The　numerical　results　of　the　drop　deformation　and　breakup　are　in　good　agreement　with　experimental　measurements.　Therefore,　the　present　model　can　be　well　applied　to　study　the　mechanism　of　the　drop　deformation　and　breakup.