Using First-principle calculations, substrate effect of O-terminated (rt3 x rt3) MgO (111) on graphene was investigated for spintronics application. Surprisingly, the graphene can be turned into a spin-polarized semiconductor, which implies that the totally spin-polarized current can be generated and its on/off switching can be also controlled. The origin of the spin-polarized band structure is spin-ordering due to alternative sp2-sp3 covalent bondings induced by the MgO (111) substrate. The results indicate that the tailored pattern of the chemisorption can be highly efficient or introducing totally spin-polarized current to the graphene.