Electrical properties of metal-mediated DNA duplexes (metallo-DNA) have been of particular interest because of their potential applications in DNA-based nanoelectronics. We prepared Hg-II-mediated DNA duplex with NH2 anchors and measured the electrical conductance of single-molecule metallo-DNA via scanning tunneling microscopy-based break junction method in the buffered solution. Three conductance values were observed that may correspond to different conformations of the metallo-DNA molecule bridged over metallic electrodes.