Using density functional calculations, we study atomic and electronic structures of monolayer hexagonal boron nitride (h-BN) and vertical graphene/h-BN heterostructures with substitutional carbon defects. For monolayer h-BN, a carbon atom substituted for a boron or nitrogen atom causes spin polarization. In the presence of an adjacent graphene sheet, however, the spin magnetic moment disappears owing to interlayer charge transfer for a C-B defect. Interestingly, two carbon defects in a hexagon of monolayer h-BN do not induce magnetic properties. In contrast, two C-B defects (two C-N defects) can have spin polarization in the vertical graphene/h-BN heterostructures because of interlayer charge transfer. (C) 2017 Elsevier B.V. All rights reserved.