Hierarchical-structure ZnO consisting of ZnO nanoneedles on ZnO nanofibers was fabricated by a two-step procedure first electrospinning and then aqueous solution processes. Field emission (FE) property of the needle-on-fiber hierarchical-structure ZnO was tested. The low turn-on field of the field emission at 4.8 V/mu m was obtained with the current density of 10 mu A/cm(2). The field enhancement factor was estimated as high as 3400. Such high FE performances of the hierarchical-structure ZnO derived from the tapered-tip structure of the ZnO nanoneedles, the high aspect ratio of the hierarchical structure and the undulant space distribution of the ZnO nanoneedles. These results would benefit the potential application of the hierarchical-structure ZnO in cold-cathode-based devices.