To the 5'-ends of two complementary oligonucleotides, 5'-mercapto-5'-deoxythymidine was introduced and reacted with the surface of either gold plate or colloidal gold particle. Well-defined Au-(nanoparticle) /DNA-duplex/Au-(plate) composite was fabricated in self-assembled monolayer (SAM) of 11-hydroxy-1 undecanethiol. The current through this composite was measured by conductive probe atomic force microscopy (CP-AFM).