The fabrication of Au-molecule-Au junctions with an Al back gate electrode structure using an electromigration method was examined. Two kinds of molecules were used for the measurements of current-voltage (I- V) characteristics: 1,4-bis[(4'-acetylthiophenyl)ethynyl]benzene (molecule 1) and 5,15-bis(3,5-di-t-butylpbenyl)-10,20-bis(4-methylthiophenyl)porphyrin (molecule 11). These molecules were adsorbed on An electrodes by self-assembly and the I- V characteristics of the molecular junctions were measured at a temperature of 5 K. Coulomb staircase-like I- V characteristics depending on the gate bias voltage were observed for junctions with molecule 11, but not for junctions with molecule 1. These molecule-dependent I- V characteristics are discussed based on the energy alignment of the junctions. We found that the experimental results could be well explained by assuming that the porphyrin core of molecule 11 acted as a Coulomb island. (c) 2005 Elsevier B.V. All rights reserved.