Using the non-equilibrium Green's function method together with the density-functional theory, the electronic transport properties of various junction models constructed from different graphene nanoribbon (GNR) units via topological defects have been evaluated. It is found that the armchair-zigzag GNR (A-Z GNR) junctions can display the diode-like behavior. Various ring structures at the interface show different effects on the transport properties and conductivity of the junctions. The simulation results suggest that the proposed models are promising for future applications in novel nano-electronics and molecular devices. (C) 2012 Elsevier B.V. All rights reserved.