ZnO nanofibers were synthesized by the electrospinning method. Their electrical transport properties were investigated by fabricating field effect transistors based on single ZnO nanofiber. The electron concentration (n) and the field-effect mobility (mu(e)) in the ZnO nanofibers are not so sensitive to the calcination atmosphere; O-2 and N-2. Importantly, n (mu(e)) in the ZnO nanofibers is bigger (much smaller) than those in single-crystalline ZnO nanowires. The existence of nanosized gains in the ZnO nanofibers is likely to be responsible for such big differences in n and mu(e). Huge grain boundaries created by nanograins seem to play the role of a source providing electrons and a barrier for electrons to flow as well. To enhance the mu(e), the calcination condition to obtain ZnO nanofibers with larger grains needs to be applied. (C) 2012 Elsevier B.V. All rights reserved.