The undoped and Al-doped ZnO nanostructures were grown on ZnO/glass substrates using a low temperature hydrothermal process. The RF-sputtering method was utilized to prepare ZnO film on the glass substrates as a seed layer for subsequently hydrothermal growth of ZnO nanostructures. The properties of ZnO nanorods were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), and room- temperature photoluminescent spectrometer (PL).The X-ray diffraction patterns show that both undoped and Al-doped ZnO nanorod arrays are grown with (002) preferred orientation. Undoped quasi-aligned ZnO nanorods were successful synthesized by this method. The current-voltage curves of Al-doped ZnO nanorod arrays exhibited a slightly higher current response than the as-grown ones. This result confirms that N-type doping of aluminum can be accomplished effectively by this method.