The uni-axially aligned ZnO nanofibers with diameters of similar to 200 nm were prepared by electrospinning followed by calcination at 600 degrees C. The effects of humidity and UV illumination on the electrical properties of electrospun ZnO nanofibers were investigated in air under different levels of relative humidity (RH). In the air with low RH, the photocurrent of the electrospun ZnO nanofiber based device increased approximately three orders of magnitude under UV illumination compared with the value measured in the dark. Intriguingly, the photocurrent increased in the air with increase of RH initially; when the RH was above similar to 50%, the photocurrent decreased gradually. In the recovery step when the UV illumination was off, a faster current decay was observed with the increase of RH. These were attributed to the competitive surface effects resulted from adsorption/desorption and the related ionization/dissociation of oxygen and water molecules on the photo-generated charge carriers.