Abstract We have developed a silicon nanowire field-effect transistor (NWFET) that allows deoxyribonucleic acid (DNA) biosensing. The nanowire (NW) was fabricated on a silicon-on-insulator wafer to provide effective ohmic contact. The NWFET sensor displayed n-channel depletion characteristics. To demonstrate the sensing capacity of the NWFET, we employed the BRAF V599E mutation gene, which correlates to the occurrence of cancers, as the target DNA sequence. The threshold voltage of the NWFET increased when the mutation gene was hybridized with the capture DNA strands on the nanowire, and decreased to the original level after de-hybridization of the gene. The shift in the drain current–gate voltage ( I D– V G) curves revealed that the electrical signal had a logarithmic relationship with respect to the concentration of the mutation gene of up to six orders of magnitude, with the detection limit in the sub-femtomolar level. The detection results of mismatched DNA sequences, including one- and five-base-mismatched DNA strands, could be distinguished from complementary DNA gene by this sensor. The excellent electrical results obtained using this label-free NWFET sensor suggest that such devices might be potentially useful tools for biological research and oncogene screening.