We investigate theoretically the electronic structure of graphene and boron nitride (BN) lateral heterostructures, which were fabricated in recent experiments. The first-principles density functional calculation demonstrates that a huge intrinsic transverse electric field can be induced in the graphene nanoribbon region, and depends sensitively on the edge configuration of the lateral heterostructure. The polarized electric field originates from the charge mismatch at the BN-graphene interfaces. This huge electric field can open a significant bang gap in graphene nanoribbon, and lead to fully spinpolarized edge states and induce half-metallic phase in the lateral BN/Graphene/BN heterostructure with proper edge configurations.