Based on space-confining effect, ZnO nanowire-based photodetectors with different diameters have been fabricated onto interdigitated electrodes with varied seed density in solution process. It is found that the rise and recovery time are determined by the diameter of nanowires due to the fact that the photogenerated carriers recombined swiftly with the increasing amount of surface states. After modifying the ZnO NWs in a hydrothermal process the response time decreases obviously, which is resulted from suppressed oxygen vacancy. A possible model about the origin of the persistent photoconductivity is proposed. The persistent photoconductivity is considered to result from the shallow donor state of neutral oxygen vacancy which is developed from electron transit to singly ionized oxygen vacancy. An electron prefers to be captured by recombination center present in middle of the forbidden band while holes are migrating to surface of nanowire caused by band bending near surface. (C) 2011 The Electrochemical Society. [DOI: 10.1149/2.086111jes] All rights reserved.