Graphene nano-ribbons junctions based electronic devices are proposed in this Letter. Nonequilibrium Green's function calculations show that nano-ribbon junctions tailored from single layer graphene with different edge shape and width can act as metal/semiconductor junctions and quantum dots can be implemented. In virtue of the possibilities of patterning monolayer graphene down to atomic precision, these structures, quite different from the previously reported two-dimensional bulk graphene or carbon nanotube devices, are expected to be used as the building blocks of the future nano-electronics.