Nanofibers(NFs), because of their high surface area and nanosized grains, have appropriate morphologies for use in chemiresistive-type sensors for gas detection applications. In this study, a highly sensitive and selective CO gas sensing material based on Au-decorated SnO2 NFs was fabricated by electrospinning. SnO2 NFs were synthesized by electrospinning and subsequently decorated with various amounts of Au nanoparticles(NPs) by sputtering; this was followed by thermal annealing. Different characterizations showed the successful formation of Au-decorated SnO2 NFs. Gas sensing tests were performed on the fabricated sensors, which showed bell-shaped sensing behavior with respect to the amount of Au decoration. The best CO sensing performance, with a response of similar to 20 for 10 ppm CO, was obtained at an optimized amount of Au (2.6 at.%). The interplay between Au and SnO2 in terms of the electronic and chemical sensitization by Au NPs is responsible for the great improvement in the CO sensing capability of pure SnO2 NFs, suggesting that Au-decorated SnO2 NFs can be a promising material for fabricating highly sensitive and selective chemiresistive-type CO gas sensors.