Based on ambient atmosphere scanning tunneling microscope (STM) technique, scanning tunneling spectroscopy (STS) combined with statistics analysis was developed to investigate the single-molecule conductance of various kinds of molecules which were self-assembled on the Au (1 1 1). Conductance histograms obtained from current-voltage curves revealed well-defined peaks at integer multiples of a fundamental conductance and were used to identify the conductance of a single molecule. The conductances of saturated molecules like 1,8-octanedithol and hexanethiocyanate were found to be 0.072 x 10(-4) G(0) and 0.06 x 10(-4) G(0) respectively and 0.23 x 10(-4) G(0) and 0.13 x 10(-4) G(0) for unsaturated molecules like 5,5'-dithiol-2,2',5',2"-terthiophene and 4,4'-dithio-tert(phenylene ethylene). (C) 2011 Elsevier B. V. All rights reserved.