A gas sensor (3) includes: a base (30), two electrodes (31, 32) formed on the base, a zinc oxide layer (34) formed on surfaces of the base and the electrodes. The zinc oxide layer includes a plurality of zinc oxide nanofibers, each having a columnar or a tubular microstructure. Preferably, the zinc oxide nanofibers are substantially parallel to each other and substantially perpendicular to the base and electrodes. Numerous apertures between adjacent zinc oxide nanofibers can retain gas molecules. If the microstructure is tubular, apertures within the zinc oxide nanofibers can also retain gas molecules. In either case, the sensitivity of the gas sensor is improved. A method is also provided.