In a carbothermal reduction process, it has been proved theoretically and experimentally that a critical limit of oxygen with addition to a conventional Ar gas is necessary to get ZnO nanostructures. More than I ppm of oxygen was required to fabricate ZnO nanowires. Related local thermodynamics and kinetics were considered and discussed. This oxygen-assisted carbothermal reduction process enables an evolution of not only ZnO nanowires but also interesting nanorods and nanosheets in series with controlling oxygen partial pressure in the range of 10(-6)-10(-2). (c) 2005 Elsevier B.V. All rights reserved.