The morphology and luminescence properties of ZnO nanowires synthesized using NiO catalyst in a chemical vapor deposition system under different growth ambient have been studied. ZnO nanostructures were prepared in nitrogen, ammonia and hydrogen ambient and characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and photoluminescence. Growth in nitrogen ambient yields ZnO nanoneedles while growth with ammonia and hydrogen ambient ends up with ZnO nanowires. Presence of the Ni tip at the end in either morphology indicated the involvement of vapor-liquid-solid growth mechanism. Enhanced green emission in ZnO nanowires implies the presence of a high density of oxygen vacancies. Influence of the ambient gases on the morphology and optical properties of ZnO nanostructures is discussed. (C) 2004 Elsevier B.V. All rights reserved.