The undoped and Al-doped ZnO nanostructures were fabricated on the ITO substrates pre-coated with ZnO seed layers using the hydrothermal method. The undoped well-aligned ZnO nanorods were synthesized. When introducing the Al dopant, ZnO shows various morphologies. The morphology of ZnO changes from aligned nanorods, tilted nanorods, nanotubes/nanorods to the nanosheets when the Al doping concentrations increase. The ZnO nanostructures were characterized by X-ray diffraction, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, photoluminescence and Raman technology. The Al doping concentrations play an important role on the morphology and optical properties of ZnO nanostructures. The possible growth mechanism of the ZnO nanostructures was discussed. (C) 2008 Elsevier B. V. All rights reserved.