We report the electrical properties of individual as-grown ZnO nanowires (NWs) on SiO2/Si and GaN/sapphire substrates. Carrier transport in metal-semiconductor-metal junction is interpreted in terms of thermionic emission and tunneling current which was assumed to dominate independently on reverse and forward contacts. Current flow in ZnO NWs grown on SiO2/Si was dominated by thermionic emission. In contrast, both thermionic emission and tunneling current contributed to the transport in ZnO NWs epitaxially grown on GaN/sapphire. Carrier concentrations of ZnO NWs were estimated to be around 8.88×1017cm-3 if grown on SiO2/Si and 1.18×1018cm-3 if grown on GaN/sapphire, respectively.