In the present study, we investigated the effect of grain size and crystallinity on the gas sensing performances of ZnO nanofibers. The grain sizes in electrospun ZnO nanofibers ranged from 20 to 80 nm in diameter for different calcination temperatures. The sensing abilities of ZnO nanofibers with different grain sizes were investigated in terms of detection of CO. In addition to the size, the crystallinity of the grains in each nanofiber influenced its sensing performances synergistically. Two domains of influence, i.e., (1) the crystallinity dominant domain and (2) the grain size dominant domain, existed. In the crystallinity-dominant domain, in which lower calcination temperatures were used, the enhancement of crystallinity overcame the adverse effect of grain growth. In contrast, at higher calcination temperatures, the adverse effect of grain growth was intensified because the enhancement of crystallinity stagnated in the grain size-dominant domain. These results strongly suggest that the simultaneous optimization of the size and crystallinity of the grains is essential to maximize the sensing abilities of oxide nanofibers. (c) 2013 Elsevier B.V. All rights reserved.