Creation of nanostructures of various sizes and shapes would be of interest in terms of realizing functional nanosystems. In this regard, we demonstrate ultrawide ZnO nanosheets together with their formation mechanism. Based on a typical carbothermal reduction process, we injected the additional O-2 gas intentionally with Ar gas. We found that the additional oxygen gas with Ar carrier gas facilitates the evolution of 2D and 3D nanostructures of ZnO. Our study on the growth mechanism of these novel nanostructures indicates that they are generated by a "1D branching and 2D filling" process. The ZnO nanosheets in this work are single crystalline, nanostructured in their thickness directions, but large enough to see with the human naked eye and to handle with tweezers. (C) 2004 Elsevier B.V. All rights reserved.