Ultrathin ZnO nanowire bundles have been synthesized on an indium-tin oxide substrate without any catalyst by using a simple thermal evaporation method, where self-organized ZnO nanowire bundles were grown on the hexagonal heads of ZnO nanocolumns. The nanowires obtained typically have diameters of 8 nm, with lengths extending to 0.25 mu m. The size is the same order of magnitude as the ZnO exciton Bohr radius (a(B)). Room-temperature photoluminescence measurement shows a prominent peak at 374 nm (3.32 eV), which is about 100 meV blue-shifted from the bulk ZnO emission.