The asymmetric current-voltage (I-V) curves are computed for a conjugated biphenyl-bipirimidine diblock oligomer with two thiol end groups sandwiched between Au(111) electrode surfaces. The method is based on density functional theory and self-consistently determines the electronic structure of the molecule coupled to the gold electrodes with varied electrochemical potentials. Spatial asymmetry in the molecule is found to be essential in generating the I-V asymmetry. (c) 2005 Elsevier B.V. All rights reserved.