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The surface effect on the photocarrier relaxation behavior using a single ZnO nanowire (NW) ultraviolet (UV) photodetector has been evaluated. The pronounced surface effect leads to the enhancement-mode field-effect-transistor behavior in dark and accounts for the slow relaxation behavior after switching off the illumination. The recovery of photocurrent is found to be strongly related to the intensity of UV light and the diameter of NWs, indicating that the photocarrier relaxation behavior is dominated by surface band bending (SBB). A model for the relaxation behavior based on the SBB of NWs is proposed to interpret the experimental results.