We report on the adjustment of the operation voltage in ZnO nanowire field effect transistors (FETs) by a simple solvent treatment. We have observed that by submerging ZnO nanowires in isopropyl alcohol (IPA), the surface of the ZnO nanowires is etched, generating surface roughness, and their defect emission peak becomes stronger. In particular, ZnO nanowire FETs before IPA treatment operate in the depletion-mode, but are converted to the enhancement-mode with a positive shift of threshold voltage after submersion in IPA. This solvent treatment can be a useful method for controlling the operation mode of ZnO nanowire FETs for wide applications of nanowire-based electronic devices and circuits.