ZnO nanowires (NWs), grown by hydrothermal and vapor phase transport (VPT) methods, were employed as the channel layers to fabricate single nanowire Field Effect Transistors (NWFETs) with a p⁺-silicon as the bottom gate. The FET employing hydrothermal grown ZnO NWs shows n-type depletion mode with a field mobility of 18.27 cm²/V⋅s, an on/off ratio of 10⁶, and a threshold voltage of -48.5 V. In comparison, the device using VPT grown NWs operates in n-type depletion mode with a field effect mobility of 36.94 cm²/V⋅s, a drain current on/off ratio of 10⁵, and a threshold voltage of -14 V. The reason for the difference of threshold voltage and the mobility by two methods was discussed in this paper.