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We demonstrate that Au-cluster-decorated single-walled carbon nanotubes (SWNTs)may be used to discriminate single nucleotide polymorphism (SNP). Nanoscale Au clusters were formed on the side walls of carbon nanotubes in a transistor geometry using electrochemical deposition. The effect of Au cluster decoration appeared as hole doping when electrical transport characteristics were examined. Thiolated single-stranded probe peptide nucleic acid (PNA) was successfully immobilized on Au clusters decorating single-walled carbon nanotube field-effect transistors (SWNT-FETs), resulting in a conductance decrease that could be explained by a decrease in Au work function upon adsorption of thiolated PNA. Although a target single-stranded DNA (ssDNA) with a single mismatch did not cause any change in electrical conductance, a clear decrease in conductance was observed with matched ssDNA, thereby showing the possibility of SNP (single nucleotide polymorphism) detection using Au-cluster-decorated SWNT-FETs. However, a power to discriminate SNP target is lost in high ionic environment. We can conclude that observed SNP discrimination in low ionic environment is due to the hampered binding of SNP target on nanoscale surfaces in low ionic conditions.