Gray-colored materials were synthesized from ball-milled ZnO powders under a thermal annealing at $1380^{circ}C$ with an argon carrier gas for 3 hours. The synthesized materials were identified to be wurtzitic hexagonal structured ZnO nanowires by X-ray diffraction and scanning electron microscopy. The ZnO nanowires have the long cylinder-like shape of which cross-section is a circle, and these nanowires are in the range 15~40nm width and 10~70 $mu m$ length, respectively. Transmission electron microscopy revealed that these nanowires are single-crystalline and grow along ［110］ direction. The optical properties of the ZnO nanowires were investigated with photoluminescence. The analytic results revealed that ZnO nanowires have the singly ionized oxygen vacancies in the surface lattices, as they emit strong green light in room temperature PL. In addition, the growth mechanism of the ZnO nanowires can be described by the vapor-solid procedures.