The negative differential resistance (NDR) effect of graphene in the p-n junctions and nanoscale barriers is investigated by using transfer-matrix method. The NDR phenomenon in the graphene p-n junctions is not so obvious as that in the conventional semiconductors, because the holes in the negative energy range also contribute to the current due to the Klein tunneling. The NDR location of graphene nanoscale barriers lies on the Fermi energy level. The block of the barrier to the current is more apparent with increasing barrier width, and the NDR effect becomes more obvious.