ZnO nanostructures, including single-crystal nanowires, nanoneedles, nanoflowers, and tubular whiskers, have been fabricated at a modestly low temperature of 550 degrees C via the oxidation of metallic Zn powder without a metal catalyst. Specific ZnO nanostructures can be obtained at a specific temperature zone in the furnace depending on the temperature and the pressure of oxygen. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) studies show that ZnO nanostructures thus prepared are single crystals with a wurtzite structure. X-ray excited optical luminescence (XEOL) from the ZnO nanostructures show noticeable morphology-dependent luminescence. Specifically, ZnO nanowires of around 15 nm in diameter emit the strongest green light. The morphology of these nanostructures, their XEOL, and the implication of the results will be discussed.