Trace of protic gases can alter the conductivity of the polycrystalline silicon nanowire (poly-Si NW) field effect transistor (FET). The aprotic gases have no effect on the electrical parameters. The proposed mechanism is due to the surface grain boundary of poly-Si NW. The direct evidence is obtained from the comparison between single- and poly-Si NW. The electrical parameters are varied according to the relative humidity levels. Nevertheless, the water molecules can be removed by vacuum indicating that it is physical absorption on the surface. As a result, the poly-Si NW FET has potential to be a reusable device for protic gas sensor.