A. step structure, very similar to a Coulomb staircase, has been successfully observed in the current-voltage (I-V) characteristic of metal/polyimide (PI)/rhodamine-dendorimer (Rh-G2)/PI/metal junctions prepared by the Langmuir-Blodgett technique. Assuming Rh-G2 molecule functions like a metallic sphere and PI works as tunneling barriers, the I V characteristic of the junctions was analyzed based on Coulomb blockade theory. Taking into account the presence of space charge at the PI-metal interface, which has been revealed by the surface potential measurement, the equation of threshold voltage of the step structure is derived. It was found that the derived equation is satisfactory to explain the I-V characteristic. (C) 2002 Elsevier Science B.V. All rights reserved.